784 The Outline of Science 



would require powerful reagents and high temperature. They are 

 not living, but they are essential to life ; they are always in a "col- 

 loidal" state, and they effect many different kinds of change, 

 though each usually does only one thing. They help in building 

 up and in breaking down. The changes they bring about take 

 place most rapidly at a certain optimum temperature, and the 

 ferments are susceptible to the influence of other substances, such 

 as salts ; some always work in couples. 



As examples of organic ferments we may mention the ptyalin 

 of the saliva that changes starch into sugar, the pepsin of the 

 stomach that changes protein into peptone, the trypsin of the 

 pancreas, and the diastase of the green leaf that alters the solid 

 starch into fluid transportable sugar. We know what these 

 organic ferments are able to do ; but we do not know what they are. 

 Indeed, there are so many unsolved problems before the investi- 

 gator of ferments, that the darkness seems almost greater than 

 the light. Facts are rapidly increasing, but a rationale of the facts 

 eludes us. 



6 



Crystals 



We have spoken of crystals ; little is known of the determin- 

 ing cause of the formation of crystals, and the subject is too com- 

 plicated a one to be dealt with in this work. The science of crystals 

 is concerned with the study of the definite geometrical forms 

 assumed by elements and compounds under certain conditions. 

 The structure of a crystal consists of a small group of atoms 

 which assume a regular arrangement in a fixed pattern. For in- 

 stance, in a crystal of rock-salt, which is built up of sodium and 

 chlorine atoms, each sodium atom has six chlorine neighbours, and 

 each chlorine atom six sodium neighbours, arranged in a manner 

 typical of rock-salt crystals. The atomic distance of these is 

 about one hundred-millionth part of an inch. 



Many chemical bodies assume the crystalline state when they 



