92 MICROSCOPIC OBJECTS. 



little weak gelatine, starch, mucus, &c. For these 

 experiments take a shde, and put one or two drops of a 

 weak solution of gelatine upon it ; then take one drop of 

 some strong solution of salts, as Epsom salts, hydroclilo- 

 rate of ammonia, sulphate of soda, tartaric acid, &c. ; 

 hold the slide over the flame of a candle until evapora- 

 tion is perceived, when it should be removed and allowed 

 to crystaUize. If the evaporation is too rapid, the crys- 

 tals will not be well formed. The main feature which 

 distinguishes these preparations is, that the crystals are 

 gracefully- curved figures, and not plain straight-lined 

 boundaries. A magnifying power of 30 diameters is 

 generally sufficient. 



Ice. — ^A plan for observing the deposition of the par- 

 ticles of water during their solidification is as follows. 

 Mix in some water a little charcoal, chalk, or other sub- 

 stance not soluble, in such a manner that a number of 

 fine particles may be mechanically suspended in it : then 

 take a large glass slide, place it on a cold night in an 

 exposed situation, as the outside of a window-sill ; then 

 pour upon it as much water as it will support without 

 running over the edge, and there let it remain all night. 

 The next morning, if the weather has been sufficiently 

 cold and the atmosphere dry, neither water nor ice will 

 be seen on the slide ; but the particles of charcoal will be 

 found arranged in the various forms which they assumed 

 while the water crystallized, and there remained after the 

 ice had evaporated. The shde thus prepared may be 

 kept for any length of time, if a slip of glass be 



