180 MICRO-CHEMISTRY. [CM. VII. 



shaken, a part of the dissolved shellac is poured into a bottle and about one fourth 

 as much gasoliu or benzin added and the two well shaken. After twenty-four 

 hours or so the flocculent, undissolved substance will separate from the shellac so- 

 lution and rise with the benzin to the top. The clear solution may then be siphoned 

 off or drawn off from the bottom if one has an aspirating bottle. (R. Hitchcock, 

 Amer. Monthly Micr. Jour., July, 1884, p. 131). 



ARRANGING AND MOUNTING MINUTE OBJECTS. 



\ 317. Minute objects like diatoms and the scales of insects may be arranged in 

 geometrical figures or in some fanciful way, either for ornament or more satisfac- 

 tory study. To do this the cover-glass is placed over the guide. This guide for 

 geometrical figures may be a net-micrometer or a series of concentric circles. In 

 order that the objects may remain in place, however, they must be fastened to the 

 cover-glass. As an adhesive substance, liquid gelatin (§ 311) thinned with an 

 equal volume of 50% acetic acid answers well. A very thin coating of this is spread 

 on the cover with a needle, or in some other way, and allowed to dry. The objects 

 are then placed on the gelatinized side of the.cover and carefully got into position 

 with a mechanical finger, made by fastening a cat's whisker in a needle holder. 

 For most of these objects a simple microscope with stand (Figs. 19, 20, 130, 131) 

 will be found of great advantage. After the objects are arranged, one breathes 

 very gently on the cover-glass to soften the gelatin. It is then allowed to dry and 

 if a suitable amount of gelatin has been used, and it has been properly moistened, 

 the objects will be found firmly anchored- In mounting, one may use Canada bal- 

 sam or mount dry on a cell ({S 232, 240). See Newcomer, Amer. Micr. Soc.'s Proc, 

 i885, p. 128 ; see also E. H. Griffith and H. 1,. Smith, Amer. Jour, of Micros., iv, 

 102, v, 87; Amer. Monthly Micr. Jour., i, 66, 107, 113. Cunningham, The Micro- 

 scope, viii, 1888, p. 237. 



MICRO-CHEMISTRY AND CRYSTALLOGRAPHY — EXPERIMENTS. 



\ 318. The student of science, and especially chemistry, so frequently requires a 

 knowledge of the appearance of minute crystals to aid in the determination of an 

 unknown substance, or for his information in studying objects where crystals are 

 liable to occur, that a few experiments have been introduced to give him a start in 

 preparing and permanently mounting some of the common crystals. 



It is recommended that the crystals be made in several ways, that is, from alco- 

 holic solutions, aqueous solutions saturated and dilute, by spontaneous drying and 

 crystallization and by rapid crystallization by the aid of heat. The modifications 

 in crystallization under these different methods of treatment are frequently very 

 striking. 



In every case the student is advised to study the appearance of the crystals in 

 the "mother liquor." As a rule, their characteristics are more clearly shown in 

 the "mother liquor" than under any other conditions. 



It is of very great advantage to examine all crystalline forms with polarized light 

 (I 209). 



I 319. Determination of the Character of the Solid Sediment in Water.— Take 

 some of the sediment from a filter or allow a considerable volume of water to stand 



