76 Aids to Microscopic Inquiry. 



obtained cohesion by another method. In the first case the 

 heat, and in the second case the mechanical force of pressure, 

 enabled the particles to come within the sphere of each 

 other's molecular attraction, and thus to unite and form one 

 body. The normal action of heat is to expand bodies, to 

 increase the. distance between their particles, and to give those 

 particles greater freedom of motion than they had before. 



The cohesion of liquids is far less than that of solids. It 

 is easy to move an iron wire through a mass of melted lead; 

 but when that lead is cooled and the mobility of its particles 

 lessened, they refuse to get out of the way of the iron, and it 

 can only be made to go through them by considerable force. 

 When we have to deal with molecules or particles of the same 

 kind, we know that they will cohere if brought near enough, 

 and when the phrase " perfect contact" is employed, it does 

 not mean that the particles really touch each other, but that 

 they approach each other very closely, and thus imitate one 

 of the conditions under which cohesion occurs. 



It often happens that we desire to join two things together 

 that we could not cause to cohere by the molecular attraction 

 of their several particles. We then introduce a third body 

 which has a strong tendency to adhere to both, and thus we 

 glue or cement them to each other. In this case we have 

 molecular cohesion between the particles of dissimilar sub- 

 stances ; the film of gum, glue, or cement strongly attracts, and 

 is strongly attracted by, the particles of the two bodies they 

 stick together. Thus molecular attraction holds together par- 

 ticles of one kind, or particles of three kinds, if the glue was 

 the means of joining two materials such as paper and glass 

 together. 



Physical attributes depend upon definite conditions. Glass 

 is not necessarily hard and brittle, but is so necessarily under 

 certain conditions. At one temperature it is like flint, at 

 another it is like putty, and at another runs like water. Certain 

 substances make sticky solutions ; but we could not say that 

 all substances which can make a sticky solution must do so 

 whatever they are dissolved in. Copal makes a sticky varnish 

 if dissolved in spirits and linseed oil, but in cajeput oil it 

 forms a jelly not more sticky than that of calPs-foot. 



The attraction which the surface of bodies exerts on other 

 substances is most important. According to Magnus* all 

 surfaces attract moisture from the air. Some surfaces do so 

 with extraordinary power. To show this, put a little bit of 

 chloride of calcium on a plate, and in a fow minutes it will 

 have taken enough moisture from the air to form a little pool. 

 Sulphuric acid attracts moisture with considerable force. Thus 

 * See Intellectual Obsebyee for June. 



