734 THE MICROSCOPE. 



glass. Another mistake, arising out of a similar want of 

 caution, may consist in confounding the effect of some 

 saline re-agent with that of the water which holds it in 

 solution. 



"2STow these remarks have a direct practical bearing on 

 the selection of tests. A preference ought to be given to 

 re-agents which leave no residue of their own : to distilled 

 water, to alcohol, ether, chloroform, benzole, and fusel oil ; 

 and to acetic acid and the dilute mineral acids. Then 

 those salts should be preferred of which the solutions 

 yield dry residues of one or two definite forms, not such as 

 put on many different shapes, are deliquescent themselves, 

 and are likely to leave moist and unstable compounds. 

 Nor is the strength of the solution a matter of little or no 

 importance ; for it should be borne in mind that the sub- 

 limates to which we apply them contain very minute frac- 

 tions of a grain; and that a very strong solution, after 

 acting on this minute quantity, would leave a coarse 

 deposit of its own, both over the general surface and at 

 the margin of the spot, which, blending with the reaction, 

 would obscure and confuse it. As a general rule, there- 

 fore, solutions of a moderate strength are to be preferred, 

 ^uch as 1 grain of carbazotic acid to 250 of water, and 1 

 grain of bichromate of potash to 100, the same of the red 

 prussiate of potash, and of the nitro-prusside of sodium." 



Other than the alkaloids and volatile metallic poisons 

 were found to yield sublimates when heated, as urea, 

 uric acid, hippuric acid, alloxan, uramile, &c; but these 

 results scarcely prepared one to expect a sublimate from a 

 blood-stain. Yet, on separating the fibres of a small spot 

 of a cotton texture stained with blood about twenty-five 

 years since, and submitting a section of the fibre an eighth 

 of an inch long to heat, a figured pattern of the colour of 

 blood was obtained, such as might be caused by a solution 

 of blood in some thin oily Liquid : this figured pattern was 

 surrounded by a colourless border, having bright figured 

 patterns such as those which mark the less characteristic 

 portions of crystalline sublimates. Dr. Guy, on repeating 

 his experiments, found the results constant ; and on con- 

 ducting them with care, and under the guidance of micro- 

 scopic examinations, two sublimates were uniformly ob- 



