NEWS

SEUM

although not the only, cause of diversity in all the ele specific gravity to be decided by the different range of the motion, and other characters by the different quality of the motion. I should prefer the first, because we have then one matter and one motion, and the simplicity is at the utmost. Diversity I suppose to be obtained, first, by interruptions in the motion, and, secondly, by collocation of atoms. But I see no reason for limiting the range by anything like a vibratory movement; that is too complex. The motion may be at first free and uncontrolled, and the direction one and unchanging. We might suppose all the atoms moving in one direction, and we should then require some mode of bringing them together. For simplicity, I shall suppose that each moves in a direct line, but that the direction of each is not the same, leaving the cause of this. And now let us try, as many others have done before us, to begin a world out of these conditions. It would be easy to say that these atoms would be attracted by each other, and so would soon rush together, but if we speak in that way we must put gravitation into the particles first, and that we have not done yet. Some one will say, Can you suppose matter to exist without gravitation? Yes, I can, and indeed my difficulty has always been to suppose it existing with gravitation, which is by no means an intellectual necessity, although a valuable property. We have only one exceedingly active atom to deal with; I say exceedingly, because it must have an irrepressible tendency (or desire) to act—one that cannot be destroyed, although its action may be controlled or put a stop to in some form. We can imagine a body like hydrogen, very active and little inclined to combine, being so broken in its spirits, to speak in metaphor, and compressed permanently, that it becomes, say, like oxygen, less disposed to move, but with more tendency to combine than before. It has given up one power for another. It has parted with activity and obtained strength and power to unite-power of attraction, power of affinity. We can still proceed, and, in our imagination, picture oxygen, or nitrogen, or any gas still more compressed-whether only mechanically or not need not now be asked,-so that it shall lose all power to move, and become a heavy metalgold or platinum. To compensate for the mobility of its youth, it has now weight; it feels that it cannot go, but it has a longing to go the tendency is irrepressible; it is the original force out of which all its properties grow. According to our ideas of the conservation of force, this is quite in order. The loss of an energy must show itself in some form, and I suppose the loss of the primitive stage of activity to be followed by the primitive stage of attraction, gravitation, exactly as the loss of the present stage of gaseous activity is followed by attraction, chemical.

HAVING begun to write on atoms, I am tempted to add a few speculations, which I trust will not be found too far removed from legitimate outlooks of reason, or too much in the region of fancy. Incapable as I am of seeing what we can do without Dalton's atom, it has been impossible to avoid thinking on the condition of things which existed before it. It has also seemed to me, as to many, that the primitive elements could not be many in number, from no inherent impossibility, but simply because of the tendency in Nature to proceed from the simple to the complex, and in doing so never to forget the past or to neglect the future. I have amused myself, therefore, by trying to make elements, as I have also amused myself by making forces to control them. I begin with one element, and I prefer the mode which I believe Graham first suggested of giving diversity. First let us read his own words from the Proceedings of the Royal Society, vol. xii., p. 620:-"It is conceivable that the various kinds of matter now recognised as different elementary substances may possess one and the same ultimate or atomic molecule existing in different conditions of movement. The essential unity of matter is in harmony with the equal action of gravity on all bodies. In the condition of gas, matter is deprived of numerous and varying properties with which it appears invested when in the form of liquid or solid. The gas exhibits only a few grand or simple features. These, again, may all be dependent on atomic or molecular mobility. Let us imagine one kind of substance only to exist ponderable matter, and, further, that matter is divisible into ultimate atoms, uniform in size and weight. We shall have one substance and a common atom. With the atom at rest, the uniformity of matter would be perfect. But the atom possesses always more or less motion, due, it must be assumed, to a primordial impulse. This motion gives rise to volume. The more rapid the movement, the greater the space occupied by the atom, somewhat as the orb of a planet widens with the degree of projectile velocity. Matter is thus made to differ only in being lighter or denser matter. The specific motion of an atom being inalienable, light matter is no longer convertible into heavy matter. In short, matter of different density forms different substances, different inconvertible elements as they have been considered. What has already been said is not meant to apply to the gaseous volumes which we have occasion to measure and practically deal with, but to a lower order of molecules or atoms. The combining atoms hitherto spoken of are, therefore, not the molecules of which the movement is sensibly affected by heat, with gaseous expansion as the result. The gaseous molecule must itself be viewed as composed of a group or system of the preceding, inferior atoms following as a unit-laws similar to those which regulate its constituent atoms. We have, indeed, carried one step backward, and applied to the lower order of atoms ideas suggested by the gaseous molecule." He afterwards says, "the motion may be assumed to reside either in separate atoms and molecules, or in a fluid medium caused to undulate. A special rate of vibration or pulsation originally imparted to a portion of the fluid medium, enlivens that portion of matter with an individual existence, and constitutes it a distinct substance or element."

This ingenious theory of Graham does not teach us how to form atoms, but we may fairly look to some earlier state of matter-something utterly "without form "-for a beginning. At present, I begin with an atom ready formed, and start from the assumption that motion is the primitive,

With this in view, let these atoms interfere with the motion of each other, and something must arise out of this interruption. The first result may be heat, but heat is not a permanent stage of a body; it is the result of interrupted activity or of work done. The inalienable tendency-the eternal quality, if we may so call it—remains exactly as before, so far as strength is concerned; but, having no outlet as before, it can only tend; that is, it can vibrate probably, or in some way aspire-it can attract or repel. This first stage of activity being now interrupted permanently, an office must be had for the power which does not die, and this we find in gravitation. It is analogous to smaller acts that occur before us.

Gravitation now begins among these interrupted atoms, and it must be powerful and crushing. In the struggle of the chaotic crowd, the fates of particles would be various. Some would in the centre be obliged very early to give up the battle, and might soon lie down as gold or platinum, no longer caring for their neighbours, but as full of real life as ever, shown by their cosmic relations, their gravitating powers. We can have any amount of diversity in this respect up to hydrogen, which still retains much of its primitive activity, but little weight--some, perhaps, like the æther of space, traversing everywhere.

In this, we suppose that a certain degree of uncontrolled activity may exist without gravitation. What is the nature of the control? Is it merely mechanical pressure, or why does a substance come down from one high stage of activity to a low stage? How is the spring broken as one may say? We could imagine vibrations when motion is interrupted, and indeed we must; and we might even seek for stages, as in musical notes and the vibration of chords, but this mechanism does not so well suit our original supposition, although it is one that must some day be properly followed out. The original supposition would suit more a colligation of atoms; first two put together, then three, &c. This would make a complete series of bodies, and we should have the various forces of Nature given out according to the repression of these atomic activities. Doubling the atom might render the action slower, and thus we keep to Graham's diversity-ofspeed theory pretty clearly.

(The power of making stages, instead of infinite gradations, is a great one, which we owe to Dalton. Whether it may ever be used to account for species is worth enquiring. At first one supposes that it is useless to look to atoms, because it would require so many to make the difference, for example, between a mammoth and an elephant. But germs are very small, and it may be that as a few atoms make a difference in organic molecules, so a few organic molecules, in a suitable position in a germ, may cause that to grow up a tiger which would otherwise have grown into a cat.)

But we must not be led too far from the object, which is to imagine a mode of obtaining gravitation, as well as diversity of atoms, with excessive motion. I need not attempt to proceed further, seeing that Grove's "Correlation of Forces" can be read. He begins with motion, and expounds with great beauty many laws, not including gravitation, however. I fear that my expositions cannot be so well proved, but it is sometimes pleasant to wander among the spheres, and to play with the mighty forces of Nature for a time. I do not remember that anyone has looked on attraction as a consequent of suspended motion, or attempted to look on gravitation as the result of diminished atomic action, producing in this way many socalled elements, but it is difficult to remember all one reads. I am also unable to recollect if anyone has separated sufficiently that activity, the suppression of which produces heat, from the eternal tendency to motion-the inherent life of the atom, and which may possibly exist as vibration within limited space in chemical compounds.

So far as I can see this matter, I say nothing opposed to any theory of heat and force now established, but, in traversing infinite space and time with abundant liberty, one is apt to take the wrong road; fortunately the way back is easy, since little depends on these visions. The illustration and prosecution of this idea may bring more solid truth, whilst attempting to prove the speculation that there is one original motion as well as one original

matter.

Атом.

RESEARCHES ON THE ATOMIC WEIGHT OF THALLIUM.*

By WILLIAM CROOKES, F.R.S., &c.

In June, 1862, and in February, 1863, I had the honour to lay before the Royal Society communications on the subject of the then newly discovered metal, thallium. In these I gave an account of its occurrence, distribution, and the method of extraction from the ore, together with its physical characteristics and chemical properties; also I discussed the position of thallium among elementary bodies, and gave a series of analytical notes.

A Paper read before the Royal Society June 20, 1872.

In the pages of the Journal of the Chemical Society, for April 1, 1864, I collated all the information then extant, both from my own researches and from those of others, introducing qualitative descriptions of an extended series of the salts of the metal. I propose in the present paper to lay before the Royal Society the details and results of experiments which have engrossed much of my spare time during the last eight years, and which consist of very laborious researches on the atomic weight of thallium. In these researches I owe much to the munificence of the Royal Society for having placed at my disposal a large sum from the Government Grant. Without this supplement to my own resources it would have been difficult for me to have carried out the investigation with such completeness.

Section I.-ON THE DETERMINATION OF ATOMIC
WEIGHTS.

In determining accurately the atomic weight of a metal that stands so high in the scale as thallium, difficulties and sources of error which are comparatively small with elements of low atomic weight are magnified to serious proportions, and require more than ordinary care for their elimination. When so large a proportion of the compound under analysis or synthesis consists of the body itself whose atomic weight is the one unknown quantity, it is evident that the almost unavoidable errors occasioned by impurity in the materials employed, the losses incident to imperfect manipulation, or the inaccuracies arising during the weighing from the omission of the corrections required by temperature, pressure, &c., will all find their way into the number which is finally considered to represent the atomic weight of the metal.

Nearly fourteen years ago, on taking the chair of the Chemical Section of the British Association at Leeds, the late Sir John Herschel called attention to the necessity which there then was for the introduction of greater accuracy in the determination of atomic weights. Speaking of the numerical relations which appear to exist between certain groups of elements, he considered that all these speculations took for granted a principle with which chemists had allowed themselves to be far too easily satisfied, viz., that all the atomic numbers are multiples of that of hydrogen. "Not until these numbers," he continues, "are determined with a precision approaching that of the elements of the planetary orbits—a precision which can leave no possible question of a tenth or a hundredth of a per cent, and in the presence of which such errors as are at present regarded tolerable in the atomic numbers of even the best determined elements shall be considered utterly inadmissible—I think can this question be settled; and when such gigantic consequences -so entire a system of nature-are to be based on a principle, nothing short of such evidence ought, I think, to be held conclusive, however seductive the theory may appear. I do not think such precision unattainable; and I think I perceive a way in which it might be attained, but one that would involve an expenditure of time, labour, and money, such as no private individual could bestow on it." Before this remarkable sentence was written, Professor Stas had commenced his classical researches on the atomic weights; and in 1861 he gave to the world the results of ten years' experiments, which had been conducted with a care and perseverance never surpassed in the history of experimental investigation. These researches of Professor Stas, and others which he has since

made public, constitute a standard of excellence which chemists who are engaged on the important task of the determination of atomic weights should strive to attain. They are, in my opinion, the most noteworthy chemical memoirs that have ever been written: not only have they determined in the most definite and unassailable manner atomic weights about which scarcely any two chemists have agreed since the time of Berzelius, but they have raised the standard of accuracy in all chemical laboratories, and have set an example which, if followed, cannot fail to

It has been with these researches before me that I have endeavoured to determine in a manner which should approach them in accuracy the atomic weight of thallium. In the determination of an atomic weight analysis is inferior to synthesis; and especially is this the case when the number sought is amongst the highest known. The method followed should be one in which as few chemical elements as possible are employed, so as to reduce to a minimum the errors arising from inaccuracy in the determination of their atomic weights,-which errors, whilst they might on the one hand balance each other, on the other might accumulate in the same direction, and become a total error of exceeding magnitude in the atomic weight of the metal under investigation. The method adopted should also be one in which there is the greatest possible difference of weight between the substance taken for the starting-point and the one ultimately obtained; for the greater the amount of this difference, other things being equal, the less likely are the unavoidable errors incidental to the method, and which may be looked upon as constant, to injuriously affect the atomic weight obtained. For these reasons processes in which a weighed quantity of the metal itself is taken and converted into one of its salts seemed likely to afford the best results; and this, accordingly, is the principal method which I have adopted.

Every substance employed in such a determination is liable to introduce errors proportionate to its own want of purity. The most extraordinary pains have therefore been taken to secure the absolute purity both of the thallium employed and of the agents used to act upon it. The glass and other apparatus have been specially constructed for these researches, andthe balances and weights have been of an accuracy never before surpassed in any research. Whilst nearly every other branch of manipulative chemistry has advanced to an accuracy vieing with astronomical observation, the operation of weighing, as almost universally carried out, is attended with grave imperfections. For ordinary analytical work, and perhaps even for more refined and accurate researches, the errors attending the ordinary process of weighing are unimportant; but in determining an equivalent so high as that of thallium no precaution whatever, which can either reduce an error to a minimum or eliminate it altogether, should be neglected. I am anxious to avoid the imputation of over-refinement in this research; but considering the fallibility of human operations, and especially those of so complicated a nature as I am about to describe, I have considered it better to err on the side of too great than of too little precaution, both in the purification of the chemicals, the arrangement of the apparatus, the time devoted to each separate determination, the removal of the errors incidental to the weighings, and the subsequent calculations. These latter have been especially tedious, as the numbers have generally extended to too many places of figures to allow the use of logarithms; each calculation has, moreover, been duplicated by different persons.

I have attempted two entirely different methods of arriving at the atomic weight of thallium. Had the results of these determinations differed materially, I should have extended the research to other methods; but as they nearly agree, it appeared unnecessary to incur so great an additional expenditure of time and material with no reasonable prospect of getting any but confirmatory results.

The first method, and that which I shall describe, consists in taking a known quantity of metallic thallium, dissolving it in nitric acid, and weighing the nitrate of thallium produced.

The second method consists in dissolving known quantities of sulphate of thallium in water, and ascertaining how much nitrate of barium is necessary to precipitate the sulphuric acid as sulphate of barium.

In the prosecution of these two methods, the materials

and the weighings, are reduced to a minimum, while several precautions have been introduced into the operations of weighing which are not usually adopted. No correction has been neglected that is not many times less than the probable error of a single observation; and, as I have stated, especially has attention been paid to such corrections as always influence in one direction, as in that for weight of air displaced. Errors, sometimes in excess and sometimes in defect, tend to disappear from the mean of a great number of observations.

I have for the foregoing reasons thought it necessary to dwell thus far upon the care I have bestowed upon my work. In the succeeding section I shall describe accurately the apparatus employed, including the balance and weights, and the necessary arrangements for weighing in vacuo. In the third section I shall enumerate the chemicals and the methods of preparing them and pure thallium. The fourth section will be devoted to the process determining the atomic weight and the weights obtained. The concluding section will consist of a calculation and discussion of results.

Section II.-APPARATUS EMPLOYED.

The absolute weight of any substance may be found by calculation from its apparent weight in an atmosphere balancing 30 inches of mercury, and from its apparent weight under, say, 25 inches of mercury; but the errors of observation, more especially those relating to the maintaining of a partial vacuum, will largely affect the result. Weighings obtained in atmospheres balancing 30 inches of mercury and 5 inches of mercury respectively will give a more accurate result; but the best weighings whereby the absolute weight of a substance may be calculated are undoubtedly one in air at ordinary pressure and temperature, and one in a highly rarefied atmosphere, it cannot be said in vacuo, owing to the difficulty of working under such a difference of pressure between the atmosphere of the balance and that surrounding it.

The Balances.

Two balances were used. That which I shall call the air-balance was made by Messrs. Keissler and Neu expressly for this work, and will clearly indicate a difference of o'0001 of a grain when loaded with 1000 grains in each pan.* It is always kept in a dry room of tolerably uniform temperature, away from draught, artificial heat, or chemical vapours, and was (in the most accurate experiments) only used when no fire had been in the room for at least twelve hours.

The second balance, which I shall call the vacuumbalance, is almost a duplicate of the first, of 14-inch beam, with agate knife-edges and planes, made by Oertling. It is enclosed in a cast-iron case connected with an airpump, and so arranged that I can readily weigh any substance in air of any desired density, the rarefaction being measured by a barometer-gauge. The accompanying diagram (Fig. 1) shows the method of the connections. The upper and lower portions of the iron case are connected by flanges and bolts; while, to ensure that the joint shall be air-tight, there is cemented to each flange a band of thick unvulcanised india-rubber, a lead wire being laid between the two pieces of india-rubber. By this means, and by causing the arm by which the riders are adjusted and the key liberating the pans and beam to work in a double-packed stuffing-box, a nearly perfect

* M. Stas employed four balances. One of them when loaded with 1000 grammes turns with 5-10ths of a milligramme; ar other when loaded with 5000 or 6000 grammes turns with I milligramme, and with 2000 or 3000 grammes in each pan turns with 3-10ths or 4-10ths of a milligramme: the third balance loaded with 5c0 grammes turns with 2-10ths of a milligramme; the fourth laden 25 grammes turns to 1-33rd of a milligramme. Reducing these weights to grains, we find thatNo. 1 loaded with 15,432 grains turns with o'0077 grain.

92.592 46,296 7, 16

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