CHEMICAL MICROSCOPY 



should always be run in comparison with any mm diameter) are used to make these micro- 

 test, until the analyst is thoroughly familiar drops and add reagent (see Clarke), 

 with crystals or any other effect that may To prevent too rapid evaporation a micro- 

 arise from the reagent alone. test is made in a hanging drop. It may be 



Test solutions can be made up of weighed put on an ordinary slide and inverted over 



quantities in a few ml or less of water, or a cavity slide. This procedure is also used 



made right on the microscope slide. The for an ordinary small test-drop (say 0.01 ml), 



analyst soon learns to judge the amount of simply as a means of preventing or greatly 



substance needed, say about 0.2 mg if there slowing evaporation and the slide can be 



is no need to conserve the substance, giving reinverted whenever rapid evaporation is de- 



with an ordinary drop of about 0.04 ml a sired. As an alternative to sealing-in, to be 



concentration of about 1:200, which is very quite sure of preventing evaporation while 



good for many microcrystal tests, usually the test stands, a drop of water, or better, 



better than more concentrated. Most of the a large "blank" test-drop, which will give 



tests are best at concentrations between virtually the same humidity as the hanging 



1 : 100 and 1 : 5000 (0.4 mg to 8 7 in 0.04 ml) ; drop, can be put in the depression of the 



the very good ones still succeed at greater cavity slide. The enclosure technique also 



dilutions, requiring only a few micrograms slows the evaporation of other substances 



even in an ordinary drop, and the tendency besides water, e.g., iodine from solutions 



is always to study and use more and more which have only a low content of iodide to 



sensitive tests. hold it in solution. 



The average analyst needs micro tests far With a standard "thin" slide (thickness 



more than "micro" procedures and equip- not over 1 mm) the usual second power 



ment, aside from the microscope itself. It is magnification (objective 20 or 21 X, or 8 



often more convenient to carry out an ex- mm) can be used through the slide. If ob- 



traction, for example, with ordinary equip- servation under still higher power is wanted, 



ment, and volumes of, say, 10 ml of solution of course it is necessary to mix the test-drop 



and 5 or 10 ml of solvent at a time, for a few on a cover-glass. In Clarke's technique the 



shake-outs, even if the amount of material cover-glass is sealed in with 25 % gum-arabic 



is quite small and the final residue micro- solution to prevent its slipping and ensure 



scopic. Often, in fact, the analyst has to only extremely slow evaporation. The seal 



process quite large amounts of material to may be made not quite complete at fii'st if 



get an extremely minute residue. Chemistry it is desired to allow a little evaporation 



has reached a stage where more and more (until precipitation occurs) before finally 



sensitivity is wanted in tests, and they are sealing. Square 25 mm cover glasses may be 



almost never too sensitive if satisfactory in most convenient, 



other respects. In new tests for basic substances in a me- 



A further long step downward can be dium of strong acid, a very small amount of 

 taken when necessary, by using small drop- solid material is put on a slide (finely pow- 

 lets. If all the substance available will make dered or in a thin deposit in one spot), a 

 only one small "macro" droplet of solution drop of reagent placed near it, and then a 

 of, say, 0.01 ml, it will still be possible to cover-glass, ordinarily of 18-mm size (round 

 try up to about 100 microcrystal tests, using or square), is added so that the reagent 

 microdrops of as little as 0.1 (mm)^ or 0.1 flows over the solid. Or, the drop of reagent 

 microliter, each containing only 1 to 0.02 is put on the cover-glass, which is then in- 

 microgram of substance, to be within the verted over the substance. The reagent con- 

 best range for most tests. Thin solid rods (1 sists of some precipitating compound in solu- 



32 



