Dr. GuY^ on Microscopic Sublimates. 9 



the same quantity ; and I may state, in illustration of the 

 great delicacy of this reaction, that on dissolving one of the 

 sublimates spoken of in this paper, which certainly did not 

 contain more than the -nri-oTrth of a grain, in the strong 

 acid, and bringing a thin line of the acid solution in contact 

 with a speck of each of the colour-developing substances in 

 turn, the characteristic rich blue, followed by the equally 

 characteristic changes of colour, took place in each instance, 

 and with marked brilliancy and distinctness in the case of the 

 permanganate of potash. Here the -rooTro^li o^ ^ grain gave 

 a distinct reaction. 



In applying this test, it is not necessary to resort to 

 the aid of the microscope. But I am now to speak of 

 two reactions in which the use of this instrument may 

 be invoked with the greatest advantage and with equal 

 confidence. The test solutions should be applied to the sub- 

 limates under the microscope, and the immediate eflPect, as 

 well as the more remote effects, carefully observed. And 

 here I would take occasion to insist on the special value of 

 the instantaneous or speedy effects of our reagents, as ob- 

 served under the microscope, in all cases in which they con- 

 sist of saline solutions. For these solutions, I need scarcely 

 observe, themselves leave crystalline deposits, especially at 

 and near the outer margin of the drop ; and it very rai^ely 

 happens that the reagent is so nicely proportioned in strength 

 and quantity as not to leave its own crystalline deposit 

 blended with that due to the reaction itself. This is one of 

 those fallacies of observation against which we cannot be too 

 much on our guard; and the reality of the danger cannot be 

 better proved than by the fact that Helwig himself, though 

 well aware that such mixed results are of common occurrence, 

 nevertheless, both in his descriptions and in more than one 

 of his photo-micrographs, shows how easy it is to neglect 

 this most obvious and familiar precaution. In order, then, 

 to guard against this fallacy, and to be able to distinguish in 

 the dry result of a reaction the appearances due to the reaction 

 and reagent resi^ectively, the first step to be taken is to pro- 

 cure, and figure for reference, the crystalline forms yielded 

 by the reagent itself; and, as I am about to treat of two re- 

 actions with the sublimates of strychnine, to which I have 

 been led to attach great importance, I will first present to 

 you the appearances worn by the reagents in question when 

 they are allowed to dry on a glass disk or slide. 



The first of these reagents — a solution of bichromate of 

 potash (t4-o) — presents, with a solution of this strength, the 

 form shown in PI. I, fig. 10. 



