solution of sodium nitrite ( per cent.) which has been sharply acidified with hydrochloric, sulphuric, or other acid. The material is now washed in cold water and stretched out on a level white surface, and exposed to light beneath the object of which it is required to produce a photograph. We may, of course, print from ordinary photographic positives, or from natural objects, or from any painted, printed, or photographed design or picture upon any sufficiently transparent material. Either sunlight or an artificial light of sufficient intensity may be employed, the time of exposure, of course, varying with the intensity of the light; in printing by daylight, the exposure varies from half a minute in bright sunshine to nearly half an hour in very dull weather.

When the decomposition of the diazo compound in the "high lights" of the picture is complete, which can easily be ascertained by means of a test slip exposed simultaneously, these portions will be found to have changed from an orange to a pale yellow. The material is now either passed into the developing bath at once, or is kept in the dark until it is convenient to develop the image. consists of a weak solution (about phenol or amine, depending upon the colour in which it is required to produce the design. The following phenols and amines amongst others have been found to give good results:

The developing bath per cent.) of a suitable

For Red-An alkaline solution of B-naphthol.

For Maroon-An alkaline solution of B-naphtholdisulphonic acid.

For Yellow-An alkaline solution of phenol.

For Orange-An alkaline solution of resorcin.

For Brown-A slightly alkaline solution of pyrogallol
or a solution of phenylenediamine hydrochloride.
For Purple-A solution of a-naphthylamine hydro-
chloride.

For Blue-A slightly acid solution of eikonogen (amido-
B-naphthol-B-sulphonate of sodium).

If it is required to produce the design in two or more colours, the respective developers, suitably thickened with starch if necessary, may be applied locally by means of a

brush or pad. After development, which with cotton is complete in less than a minute, the material is washed, and the picture requires no further fixing. In the case of the purple and blue developers it is necessary to wash the material finally in a very weak solution of tartaric acid.

Velveteen, linen, silk, wool, and other fabrics are treated in the same way, except that in the case of wool and silk a longer exposure to light is necessary, and the immersion in the nitrite and developing baths must also be more prolonged. The maroon and blue developers are not suitable for silk and wool.

Architectural and engineering drawings can be copied with clearness and accuracy upon paper or calico which has been dyed with primuline, diazotised, and dried. The copy is developed by brushing over the surface a solution of Bnaphthol or a-naphthylamine.

Transparent photographic pictures can be produced upon glass or celluloid by coating them with a film of gelatin containing primuline, then diazotising with nitrous acid, printing, say from a carbon or silver positive, and developing as before.

In point of simplicity the process leaves nothing to be desired; and in regard to cost, offers considerable advantages over any known process of photographic printing.

So far as I am aware, it is the first process which has been devised in which derivatives of the aromatic hydrocarbons are employed for the production of a positive image capable of development in colour. In this respect it differs altogether from that recently published by A. Feer. This process depends upon the sensitiveness to light of the diazo-sulphonates, produced by the action of the inorganic sulphites upon diazo compounds.

The action of the light consists in a dissociation of the diazo-sulphonate, the diazo group being set free. If, therefore, a mixture of such a compound with a phenol capable of reacting with the diazo derivative (to form a colouring matter), be exposed to light, synthetic union takes place between the diazo group and the phenol, in proportion as the former is set free; and if, therefore, the exposure takes place as in the

ordinary process of photographic printing, a coloured image or picture results. The product of the union being a colouring matter sufficiently insoluble to resist washing, the unaltered mixture of diazo-sulphonate and phenol being, on the other hand, freely soluble, the print is finally fixed by copious washing. By this method it is obvious that a negative is produced, and is fully developed in and by the printing process, whereas, in the primuline process, the light acting as a destructive, not constructive agent, the exactly opposite result is produced, and the development of the image is a subsequent operation.

In comparing the diazotype process with the ordinary methods of photographic printing, the next question which arises is as to the mode of action of the light, considered (a) physically, and (b) chemically.

(a) The rays which decompose the diazotised primuline are different in order and distribution from those which attack silver bromide. Photographs of the spectrum by the two processes taken side by side show, in the case of the diazo print, a sensible shifting of the area of maximum effect towards the red.

(b) In regard to the chemical changes which accompany the "discharge" of the diazo-primuline on exposure, the main point which has been established is, that the diazo group (-N=N-) is completely eliminated as free nitrogen. The evolution of gas is easily observed by allowing the light to act upon the sensitive surface submerged in water; and quantitative observations showed that the volume of gas evolved was in accordance with the requirements of the formula, on the hypothesis that the diazo-nitrogen was split off. This result is confirmed by the observation that the product of decomposition is not re-converted into primuline, nor into the corresponding hydrazine by treatment with. sodium hyposulphite ("hydrosulphite"), or with stannous chloride. As to the constitution of the product, it remains undecided whether it is the phenol corresponding to the diazo-primuline, the decomposition being analogous to that of the diazo-benzene by heat, or whether the primuline residue enters the molecule of the cellulose or other medium which

carries it. The latter hypothesis is supported by the fact already mentioned, that the molecular union of the diazoprimuline with the colloid medium is a necessary condition of the reaction, for the free diazo-primuline when exposed to light in a thin film is either not decomposed at all or only after very prolonged exposure.

Mr E. W. Foxlee has found that pictures can be produced by causing heated engraved blocks to act upon paper or cloth treated with diazotised primuline. Those parts in contact with the heated surface are decomposed and are therefore no longer capable of combining with any of the developers.

Explosives as used in Rock-blasting. By ALEXANDER CLARK, Kirkcaldy.*

Rock-blasting may have for its object simply the removal of the stone without regard to any use to be made of it, as in the case of a railway cutting or the excavation for a dock or harbour. If the material be very dense and hard to pierce by boring, it will be found most economical to use a powerful explosive, which will require smaller bore-holes, and will effectually shatter the rock into fragments, so that it may be more easily removed. Besides getting the maximum effect with such an explosive on the portion actually removed, the adjoining rock will be considerably affected if the explosive has been sufficiently confined.

If, again, the material to be excavated is of a soft and friable composition, such as schist, blaize, boulder clay, or such like, it will give a much better result to use a less violent explosive, which will have a lifting more than a breaking effect. In this case a large bore-hole with a heavier charge of a slow combustion powder will have much more effect than a charge of an instantaneous explosive of equal power.

Again, in excavating rocks, there may be an object in having the pieces as large as possible, or of useful sizes for

* Read before the Society, 22nd February 1892.

special purposes. Such are most quarrying operations for monumental or building stones, or works in railway or dock cuttings, where the stone is of good quality and serviceable for constructive works. In cases such as these, great care requires to be exercised, so that the stone may not be broken by too heavy explosions; the holes bored for explosives are placed as near as possible to some natural joint, or where this cannot be got at, the weakest point in the rock, where a small shot may break up the way for a larger explosion to follow. Very often it is necessary, even where a large quantity of rock is to be removed, to put in a small charge to open out the joint, so that a larger quantity may be used again in the same hole for the complete removal of the rock. A large piece of rock may thus sometimes be shaken repeatedly before the final charge is put in. In any operations such as these, strong and violent explosives are quite inadmissible; slow combustion is absolutely necessary if the rock is to be preserved as nearly as possible without flaw. The quarryman generally. finds that the rock has naturally more flaws than he would

care to see.

Further, besides considering the nature of the materials to be excavated and the objects to which they are to be applied, it is necessary to consider if the rock is water-bearing, as some explosives only can be used in wet places without special provision for their protection. It may thus be necessary to use some explosives entirely under water: some are rendered inoperative by damp, some require special provision for ignition or detonation, while others act in water in exactly the same way as in a dry position.

Again, owing to the varieties in the forms of rock excavation it will be very difficult to arrive at any general rule as to the quantity of explosive material required for a given amount of rock. A large area, such as a dock excavation having abundant space on at least two sides, may require only a small weight of explosive proportionately with a narrow railway cutting, and this again will consume much less explosive material proportionately with a cutting in a small sewer trench. If the cutting be worked so that the rock slips naturally off its bed, it will also require less