ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 725 



point of a penknife or of scissors : a drop of green liquid will exude. 

 Put some of this on each slide, spreading it out a littlo so that it shall 

 not be too thick. Place the slides in a temperature of about 70° F., 

 and put over them a piece of paper to exclude light and dust. In 

 about half an hour they should be dry, and, if successful, the crystals, 

 few or many, should be formed. Mount with balsam. If the crystals 

 are urea, early mounting is advisable, since their easy solubility 

 might put them in peril, on account of the natural moisture in the air. 

 If they prove to be urates, which are not so soluble, the mounting 

 can, if necessary, be deferred. 



Preparing Micro-crystals.* — Dr. K. Haushofer points out that, 

 although it is useful to produce microscopical preparations for the 

 purpose of comparison and demonstration, yet it should not be for- 

 gotten that a single precipitate or a single crystallization rarely 

 shows all the important forms of a compound, and that, as a rule, the 

 same compound has to be prepared several times, under different 

 conditions, if we desire to obtain a perfect standard of comparison. 

 If we neglect these precautions, and rely merely on a single pre- 

 paration, we shall occasionally arrive at incorrect judgments. 



Many of the chemical compounds are quite unsuited for per- 

 manent preparations, as, for example, many salts of silver, mercury, 

 and lead, the majority of the carbon compounds, &c. In most cases 

 it is found to be a more efficacious plan to put up the crystals dry, 

 and protected against dust by a cover-glass fixed by Canada balsam, 

 than to imbed them in a resinous medium. If the nature of the 

 preparation permits, care should be taken to wash away any 

 secondary crystals which might obstruct observation, and also any 

 residues from the precipitant, or from the original solution. This 

 is very often favoured by the circumstance that the micro-crystals of 

 a precipitate adhere pretty firmly to the slide in which the reaction 

 has taken place, and especially when precipitated by heat. It is 

 then merely necessary to put the slide in a sloping position in a 

 vessel of water, and having withdrawn it with care, to allow it to 

 dry in an almost vertical position. If the precipitate does not adhere 

 firmly enough to the slide, the latter is placed in a large test-tube, 

 water is poured over it, and the precipitate allowed to subside. 

 Every drop of the water may be removed by decanting. The 

 precipitate is then placed on a slide, and left just as it is, or the 

 greater part of the water removed by the aid of blotting-paper. Of 

 course, only quite insoluble precipitates tolerate washing without 

 injury to the crystals. Very slight degrees of solubility are recog- 

 nizable by a roughening of the crystalline surfaces. 



Micro-chemical Demonstration of Albivmen.f — Dr. 0. Loew has 

 employed two tests for albumen, viz. the Berlin blue test and the 

 biuret test. In the Berlin blue reaction the preparations were 



* Haushofer's ' Mikroskopische Reactiouen,' 1885, pp. 1G1-2. 

 t But. Ztg., xlii. (1884) p. 278. 



