CH. IX] MATERIAL. 223 



constituents are made with differently treated portions 

 of the original substance, it is better to calculate all 

 results in p.c. of material dried at 100°, i.e. in p.c. of 

 dry weight. 



The moisture (i.e. loss on drying at 100°) having been 

 determined, the calculation from one state to the other 

 involves very little trouble. 



Before treatment with any solvent the substance 

 should always be as completely disintegrated as possible 

 — this is a point which requires very careful attention 

 and too much importance cannot be given to it. 



Ordinary dry tissues, such as leaves, portions of her- 

 baceous stems, etc. can generally be reduced to a fine 

 powder without much trouble, but hard tissues are often 

 difficult to extract satisfactorily. Unless very tough — 

 which will seldom be the case — grinding in a small mill, 

 such as is used for grinding coffee berries, will generally 

 bring the substance into a suitable condition. 



Fresh tissues are more troublesome to work with : 

 they can be treated in a large mortar with a small quan- 

 tity of the solvent, and thus rubbed up into a perfectly 

 homogeneous paste. The addition of some sand, or very 

 finely powdered glass, facilitates the process and is seldom 

 objectionable: indeed its use, in preventing the solid 

 material 'caking' during extraction, often more than 

 counterbalances the inconvenient increase in bulk which 

 it causes. If it is desirable to weigh the residue at the 

 end of the successive extractions an exactly weighed 

 quantity of sand or glass can be used and its weight 

 deducted from that of the total residue. 



