ELECTRICITY FOR BLASTING 100 



positions, which had become damp by exposure to air, led to a trial of the effect of 

 moisture in promoting the ignition of but slightly sensitive compositions ; and it was 

 ultimately found that the impregnation of ordinary gunpowder with a small amount of 

 moisture (by an expedient similar in principle to one adopted with considerable success 

 by Captain Scott, R.E., in connection with charges to be fired by the induction coil- 

 machine), rendered its ignition by means of the magnet a matter of certainty. 



Some important precautions were, however, indispensable to the attainment of this 

 definite result. If the slightly-damp powder was employed in a finely-divided con- 

 dition, it very frequently became caked between the wire-terminals in the fuze, and 

 the current would then pass through the composition without igniting it. This was 

 found to take place occasionally, even when the powder was employed in its original 

 granular condition. Several attempts were made to overcome this difficulty by 

 modifying the form and position of the terminals or poles in the fuze, and I at last 

 contrived a perfectly successful arrangement, in which only the sectional surfaces of 

 the terminals, consisting of fine copper-wire (0'022 inch diameter), were exposed to 

 the interior of the fuze (see a, fig. 782), so as not to project at all. The prepared 

 gunpowder, therefore, simply rested upon the surfaces, and a perfect uniformity in 

 the action of the fuze was attained. The priming composition consisted of fine- 

 grain gunpowder, which had been soaked in an alcoholic solution of chloride of 

 calcium, of a strength sufficient to impregnate the grains with from one to two per 

 cent, of that salt. The prepared powder was exposed to the air for a short time, to 

 permit of a sufficient absorption of moisture by the deliquescent salt. Upwards of 

 500 quill-fuzes (of the description employed for firing guns), primed with the pre- 

 pared gunpowder, and fitted with the arrangement of the terminals above referred to 

 (fig- 782), were fired with the larger lever-magnet. The failures 

 did not amount to more than 3 per cent., and were all proved to 

 be due to defective manufacture. In the experiments with these 

 fuzes, one or two simple rheotomic arrangements were success- 

 fully employed for effecting the rapidly successive discharge of 

 a series of fuzes. 



The above fuze was found to be easy of manufacture and 

 permanently effective. While, however, it presented a certain 

 means of effecting the ignition by the aid of a powerful magnet, 

 of single charges, or of a large number, to be fired in moderately 

 rapid succession, it was inapplicable to the ignition with cer- 

 tainty of more than one charge in circuit. After a great number 

 of experiments, I at length succeeded in the production of a 

 priming material for the fuze, which greatly exceeded in sensitive- 

 ness any of the other compositions hitherto tried. 



The new priming composition consisted of a very intimate mixture of sub- 

 phosphide of copper, chlorate of potassa, and levigated coke, the latter substance 

 being employed to add to the conducting power of the mixture, which was found 

 otherwise insufficient. 



In the course of experiments subsequently carried on with fuzes which contained 

 this composition, it was found that a slight residue, consisting principally of the 

 coke employed, occasionally remained on the surfaces of the terminals in the fuze, 

 after its discharge, and by forming a good conducting link between them, interfered 

 with any future effects of the magnetic current in other directions, by the establishment 

 of a complete circuit. This obstacle to the perfect success of the composition was 

 entirely removed by the substitution, for the coke, of another material, more easily 

 acted on by the chlorate of potassa, and answering equally well as a conducting 

 medium ; namely, the sub-sulphide of copper. No instance has occurred in the dis- 

 charge of several thousand fuzes, primed with the mixture of sub-phosphide and sub- 

 sulphide of copper with chlorate of potassa, in which the terminals have not been 

 found quite free from adherent residue, after the ignition. 



The sub-phosphide of copper, which is produced at an elevated temperature, is a 

 compound of very stable character, and the mixture of the three constituents is quite 

 as unalterable as the explosive mixtures which are in general use for the preparation 

 of percussion caps, &c. The stability of the mixture has already been submitted to 

 very satisfactory tests. Fuzes primed with it have been found to have lost none of 

 their delicacy and certainty, when tried more than two years after preparation. 



The sub-phosphide of copper, intimately blended with chlorate of potassa, forms a 

 mixture in a high degree sensitive to the effect of heat, and possessed, at the same 

 time, of some power of conducting electricity. With the employment, however, of 

 magneto-electric machines of comparatively low power, and in cases where the re- 

 sistance to be overcome by the current is considerable, this conducting property is 

 not sufficient to insure the ignition of the mixture by assisting the passage of the 



