APPENDIX 149 



Instruments for determining the specific gravity of 

 liquids are called hydrometers. They are made of glass 

 with weight in the bottom, a bulb in the middle, and a 

 stem containing a scale at the top. When placed in a 

 liquid they sink to a certain level, and the specific 

 gravity is then read from the scale. Specific names are 

 given modifications of these instruments intended for 

 definite liquids, as urinometer, lactometer, etc., for urine 

 and milk. 



Cohesion and Adhesion. Cohesion is the force by 

 which the particles of matter of the same kind are held 

 together. The particles of a bar of iron for instance 

 cohere or are held together by cohesion. Adhesion is 

 that form of energy by which masses of matter of dif- 

 ferent kinds are held together, as when a carpenter 

 joins two pieces of wood with glue. It is obvious that 

 cohesive energy must be exerted in greatly different 

 measure in different masses. It is powerfully shown 

 in the bar of iron and inappreciably in a volume of 

 water. Cohesion, therefore, acts not only at immeasur- 

 ably small distances, but with such varying force as to 

 determine the hardness of the diamond or the softness 

 of wax. Adhesion is sometimes employed to remove 

 one substance from another, as in the process of filtra- 

 tion through charcoal to remove coloring matter. Matter 

 is said to be hard when it requires great force to scratch 

 it ; brittle, when it breaks only from violent blows ; tena- 

 cious, when it resists stretching; malleable when it can be 

 beaten into thin sheets, and ductile when it can be pulled 

 out into fine strands, like wire. 



Crystals are solid substances bounded by plane faces 

 and definite angles (Bliss and Olive). The process of 

 forming crystals is called crystallization. 



