8i8 BOTANY AND PHARMACOGNOSY. 



crystals are formed and not aggregates or groups. These 

 isolated crystals can then be studied independently. The reason 

 why aggregates are formed is because the crystals are per- 

 mitted to grow too rapidly on the slide. This is usually the case 

 in the usual method of procedure in securing crystals, i.e., by 

 adding a drop of a solution to the slide, and then allowing it to 

 evaporate spontaneously, under ordinary conditions. If on the 

 other hand the rate of evaporation is lessened so that there is a 

 slowing down of the growth of the crystals, individuals may be 

 obtained of almost any size desired. And it will be found that 

 these isolated crystals may be quite as easily prepared as the 

 aggregates which seem so characteristic to the average student. 

 Special methods, however, may be necessary to obtain such 

 isolated crystals. For instance, single crystals of menthol ( Fig. 

 ^Tiy, A) arc obtained by means of sublimation rather than from 

 solutions. Cumarin crystals (Fig. 131) are easily obtained 

 by controlling the temperature of the melted mass, etc. 



The interest in these crystalline substances is becoming greater 

 as foods and drugs and technical products are subject to stand- 

 ards of purity. Most of the crystalline constituents common to 

 plant products are usually said to be calcium oxalate. This 

 substance is insoluble in water, alcohol, and acetic acid, soluble 

 in the mineral acids and occurs usuall\' in definite crystals. 

 These crystals are rather easily studied in Iris (Fig. 320), 

 Ouillaja (Fig. 281, C) etc. (see page 170). They are found 

 to crystallize either in the tetragonal or monoclinic systems, 

 sphenoids of the latter being present in Belladonna (Fig. 281, 

 D), Scopola .(Fig. 175, a), etc. 



Some substances occur in a crystalline form even upon the 

 commercial product as vanillin upon vanilla pods and cumarin 

 upon tonka seeds ; or crystals may be found in special cells 

 as in Piper album and Piper nigrum. In alcoholic material 

 particularly of fresh drugs characteristic sphero-crystals are 

 found as in Inula (Fig. 182). Sometimes similar sphero- 

 crystals are observed upon soaking the drug of commerce in 

 water and then adding alcohol as in Scilla. Again crystalline 

 substances separate upon the addition of mineral acids, as when 

 nitric acid or sulphuric acid is added to sections of Hvdrastis 



