20 



but especially if it be kept in tlie liyht, the result being similar to the 

 foregoing. The third substance, pyroxj'line, (explosive gun cotton), is 

 much more stable, and does not decompose under ordinan* conditions. 

 These experiments I can confinn from my own observation ; and, 

 coupled with the fact that the sensitiveness in some measmre varies with 

 the solvent, we have ample grounds for assuming that the reactions 

 involved depend largely on collodion itself as a reducing agent. 



These considerations derive especial interest from the case of Talbot 

 V. Laroche, recently decided in the Court of Common Pleas, in which 

 the plaintiff sought to restrain the defendant from practising the collo- 

 dion process, in virtue of his patent for the paper process. In this case, 

 as in the celebrated " coal case," the scientific evidence was very con- 

 llicting, and high authorities held opposite opinions. None can read 

 the account of this trial without feeling great interest in the course of 

 the enquiry, and perceiving that great legal and scientific principles are 

 involved in the decision ; and although in the course of the tiial these 

 may aj)pear at issue, the summing-up of Chief Justice Jervis on the 

 case exhibits a remarkably clear, intelligent, and comprehensive \ie\y 

 of the question, which in itself must be considered a difficult and 

 obscure one. The claim in the patent of Mr. Fox Talbot, as limited 

 by the Court, amounts only to the use of gallo-nitrate of silver as a 

 sensitive agent, applied to paper previously prepared with iodide of 

 silver, and the use of the same compound appUed as a developing agent, 

 and lastly, the use of a soluble bromide as a fixing agent. Under this 

 construction the jury found for the defendant. The claim, however, 

 advanced by the plaintiff under his patent, amounted to the general 

 use of salts of silver, developed by reducing agents and fixed by chemical 

 solvents, which of course would include every loiown efficient process. 

 This claim is based upon the principle that " chemical equivalents are 

 infringements of patents," and that the employment of any known 

 chemical to effect the same result as that specified in the patent, would 

 infringe the patent, by virtually using the same means, just as 

 a mechanical equivalent of power is protected under a patent right. 

 With this view it was argued that the collodion is a mechanical equiva- 

 lent for the paper, and simply serves as a surface for the precipitation 

 of the silver iodide ; that pyrogallic acid is aUied to gallic, and both 

 perform the same office in the reduction of silver ; and that the choice 

 of a solvent or fixing agent is comparatively unimportant : hence that 

 the processes were identical m principle, and nearly related in practice. 

 Against this view it may be urged, that considering the nature of colic- 



