Dr. Gu\% on Microscopic Sublimates. 3 



analogous bodies. It is now proposed to apply this test of 

 heat in such a way that not only shall the direct changes of 

 form, colour, and position be noted, but the deposit from the 

 vapour or smoke be collected and examined, and then sub- 

 mitted to the action of reagents. So that to the one test of 

 heat the two important subsidiary tests of the microscopic 

 character of the sublimate, and that of its reactions, are 

 superadded, the three together constituting a compound test, 

 or method of procedure, obviously admitting of most extensive 

 application. Indeed, if we reflect on the number of distinct 

 elements which a full description of the results of this com- 

 pound test, as applied to a minute particle of any solid body, 

 or to the deposit from a solution, must involve, it will be 

 obvious that there are very few, if any, substances volatile or 

 decomposable by heat, which by its means we should fail to 

 identify. This result would be still more certain if we first 

 submitted the substance to microscopic examination. 



3. Practical utility.— ^o turn this simple method of pro- 

 cedure to practical account in chemistry and toxicology, three 

 things are necessary. The results obtained should be cha- 

 racteristic ; the quantities which yield them should be ex- 

 tremely small ; and the method should admit of application, 

 not only to the substance itself, but to the deposit from its 

 solutions. All these conditions are fully satisfied, not only in 

 the case of such simple matters as arsenious acid and corro- 

 sive sublimate, but also in the cases of the principal poisonous 

 alkaloids, such as strychnine, morphine, and veratrine. I 

 will illustrate these three conditions by instances in point. 



As examples of characteristic changes of form due to the 

 application of heat, I may instance the complete dispersion 

 in white vapour of arsenious acid and corrosive sublimate ; 

 the change of colour, melting, fuming, and deposit of carbon, 

 which mark the alkaloids as a class ; the deposit of carbon 

 and reduction of silver from the tartrate of silver ; the ex- 

 plosion of the oxalate of silver; and the quick rosy dis- 

 coloration of alloxan. As examples of characteristic sub- 

 limates, I may mention the brilliant octohedral crystals of 

 arsenious acid, contrasted with the radiating and projecting 

 groups of needles of corrosive sublimate ; the jointed plates 

 and prisms of cantharadine ; the crossed twigs of solanine ; 

 the detached rhomboidal crystals of veratrine ; and the com- 

 pound crystals and radiating patterns of strychnine, mor- 

 phine, cryptopia, &c. As examples of characteristic reactions 

 I may specify that of morphine with distilled water, and with 

 dilute hydrochloric acid ; and those of strychnine with the 

 solutions of bichromate of potash and carbazotic acid. 



