are stated in an economical form, with the cost attached, as in the following table:

Weight and cost of various articles of food required to be consumed in the body, in order to raise 140 pounds to the height of 10.000 feet.

The position occupied by oatmeal as a cheap source of force shows that its extensive use appears thus founded on sound economical principles. Certainly few foods can compete with it in respect to its capability of affording support to the bodily functions at a minimum of expense. The comparative small force-value of animal foods, combined with their high price, render them (according to the foregoing table) most uneconomical articles of diet. It should, however, be remembered, the relative digestibility of the various foods is a

condition of considerable importance, not only on dietetic grounds, but also from purely mechanical considerations. It has been mentioned above that the labour of digestion is itself a considerable source of internal work, though we are ignorant of its amount. It may therefore frequently happen that the ready digestibility of an article of food may more than compensate for its otherwise inferior value as a source of force.

MODERN EXPLOSIVE COMPOUNDS.

NITRO-GLYCERINE, ITS PERILS, AND ITS PROBABLE USES.

THE explosive compounds with which chemistry has furnished us may very conveniently be divided into two classes, only one of which, however, is likely ever to be applied to practical purposes. The explosion produced by compounds belonging to one of these classes is due to the weak affinity which exists between their constituent elements; and the ease with which, therefore, they suddenly and almost spontaneously separate. A liquid or solid being changed instantaneously, and with uncontrollable energy into gases, which occupy many times a larger space than before. Peroxyde of hydrogen (H2 O2), and perchloride of nitrogen (NC), are striking examples of this class. The former being decomposed with explosive violence, by mere elevation of temperature, into water and oxygen, on account of the very weak affinity of the second atom of oxygen; and the latter, when dry, by the slightest contact, or the least motion, into chlorine and nitrogen.

Bodies belonging to the other class explode from a very different cause; the result produced in their case being nothing more than a very rapid combustion, due to the presence and complete admixture of the combustible and supporter of combustion. The most appropriate illustrations of this class are found in gunpowder, gun-cotton, and nitro-glycerine. That these are well suited for producing a very rapid combustion is evident from their constitution, which is as follows:

This formula for gunpowder is what has been found by theory, but it corresponds very nearly with that of the best The constitution, both of gun-cotton and nitro

kinds.

glycerine, is as yet uncertain; but it is most probably very nearly the above. The presence of sulphur in the gunpowder is indispensable; but it is objectionable for many reasons. war it fouls and corrodes firearms; and in both war and mining it produces noxious gases. And hence, one of the advantages, both of gun-cotton and nitro-glycerine, is that the combination of their elements, during their explosion, gives rise to no corrosive or suffocating compound; nor does either of them leave any residue.

The resemblance between the constitution of these three important bodies is very striking. In all of them nitric acid, which is extremely rich in oxygen, and is very easily decomposed, supplies the supporter. Each of them has, however, its advantages and disadvantages. Gunpowder, for example, is not liable to spontaneous decomposition, but it is spoiled by moisture. Exactly the reverse is the case with nitro

glycerine.

On the present occasion we shall restrict our observations to nitro-glycerine, as it is now exciting considerable attention, and is likely very soon to become, almost exclusively, the explosive agent in mining operations. "Nitro-glycerine," called also" Glonoine," "Nobel's blasting oil," etc., was discovered in 1847 by Sobrero, a pupil of M. Pelouze, who, by acting on glycerine with a mixture of two volumes of sulphuric acid, and one volume nitric acid, and then adding water, obtained a yellowish, oil-like compound, which was inodorous, of a sweetish pungent taste, producing headache, when merely applied to the tongue, and highly explosive. This remarkable compound for a considerable time attracted but little notice. It was long used only as a medicine, and almost exclusively by the homoeopathists; and, strange to say, considering its violent properties, was prescribed as a cure for nervous affection.

It is merely a compound of nitric acid and glycerine. The latter, though so energetic in combination, is one of the mildest substances known. It was discovered by Scheele in 1779; and, from its extreme sweetness, was termed by him the "sweet principle of oils," a peculiarity indicated by its present name. It ranks as an alcohol, since its formula may be considered as CHO. Combined with the fat acids, it affords, in the fixed oils and fats, compound ethers. In saponification the acid of the compound ether unites with the inorganic base, the glycerine being set free. It forms a great number of compounds, in some of which it is combined unchanged; thus in the case of the fat acids; in others, the monatomic radical acid takes the place of one or more atoms of its hydrogen.

It is produced during the alcoholic fermentation of sugar, and it occurs in all fermented liquors, but particularly in wine. It may be formed artificially with great ease.

When glycerine is acted on by nitric acid alone, and at ordinary temperature, oxalic acid is the result; but when submitted to the action of a mixture of nitric acid and sulphuric acid at a low temperature, three atoms of the nitric acid combine with one atom of the glycerine, nitro-glycerine being formed.

Nitro-glycerine is heavier than water, its specific gravity being 16. It is insoluble in water, but dissolves in alcohol and ether. Its insolubility in water renders it extremely valuable for subaqueous purposes. Possibly it may be found well adapted for torpedoes. It freezes at about 46° Fahr., and explodes at about 350°. Flame will not always ignite it; and when it is struck with a hammer on an anvil, only the portion which is actually struck explodes. It is exploded, however, by agitation or friction-when frozen, by the friction even of its own particles; a very serious source of danger, as it freezes at very moderate temperature. Considering its resemblance to gun-cotton, and the circumstances under which it sometimes ignites, there is but too good reason to believe that it is liable to spontaneous decomposition, which might easily give rise to a temperature sufficiently high to cause ignition. It is probable that the oxygen of the nitric acid which it contains slowly combines with its hydrogen and carbon. In such a case, heat would be generated, and, if not allowed to escape, would accumulate to a perilous or even fatal extent. One of the greatest dangers that accompany nitroglycerine arises from the fact that it can be made only at a low temperature, and that it explodes if the temperature is very moderately augmented. Nevertheless, were it not liable to spontaneous decomposition, it would be at least as safe as gun-cotton. Gun-cotton often explodes from a similar cause. If it is slightly acid, fumes will be generated by the reaction of the acid; and if these fumes are not allowed to escape, and especially if the heat is retained by means of non-conducting bodies, the temperature will become high enough to cause ignition. Such a result is extremely likely to take place if the gun-cotton is placed in a bottle, which is surrounded by sawdust contained in a wooden box. Many explosions that have puzzled photographers are explained in this way; and there is reason to believe that nitro-glycerine in wooden boxes has exploded from similar causes. The composition of nitroglycerine being so similar to that of gun-cotton, it is not wonderful that it will ignite in similar circumstances.

Nitro-glycerine is supposed to be eight times as powerful

as gunpowder, weight for weight, and thirteen times as powerful, bulk for bulk. This renders it, though dearer than gunpowder, more economical for mining purposes, on account of the diminished labour in boring. It requires no tamping; the hole in which it is placed, and which may without inconvenience be damp, may be merely closed with water or sand. If there are any crevices, they must of course be stopped, or the oil will flow away, and be wasted; but damp clay will answer well for the purpose.

Various experiments have been made, with the object of testing the explosive power of nitro-glycerine, some of them for the purpose of comparing it with gunpowder. Thus a hole fifteen inches deep was bored within eighteen inches of the edge of a large block of calcareous stone; and a charge of oil one and a half inches deep was placed in it, after which it was closed with plastic clay. The explosion caused the block to be blown into small fragments; a similar charge of powder broke a similar block into only three pieces. A hole three feet deep and one and a quarter inches in diameter was bored in a solid rock three feet from its face, and three ounces of nitroglycerine were placed in it, and covered with a wad of paper, on which water was poured. The explosion caused a displacement to the depth of six feet. In a subsequent experiment the hole, which was eight feet two inches deep, and half an inch in diameter, was bored nine feet from the face of the rock, and was charged with oil to the depth of eighteen inches. The explosion completely separated one hundred tons of the rock. There is reason to believe that half a hundredweight of gunpowder would be required to produce the same effect.

The wonderful capabilities of nitro-glycerine as an explosive agent have been hitherto, unfortunately, far more than counterbalanced by the fearful damage with which its use is accompanied. Already some appalling catastrophes have been produced by it; and the recklessness with which it is carried from place to place, and the temptation there is, from motives of economy, to pass it as ordinary merchandize, greatly enhances the peril; so that a stringent parliamentary enactment regarding it is loudly and somewhat generally demanded. That legislation, to some extent, has become necessary, can scarcely be denied, the destruction of life and property by means of this terrible agent, having been already so considerable. In November, 1865, an explosion of nitro-glycerine occurred in Greenwich Street, New York. It was caused by a quantity contained in a small rudely-formed box, which was found in the baggage-room of the Wyoming Hotel. Red fumes of an offensive smell being observed to issue from the