42 AN INTRODUCTION TO SCIENCE 



one in five or one in ten and thus save unnecessary labour in 

 their examination. A worm an inch long may be cut into 

 thirty thousand sections by an assistant who has no knowl- 

 edge of anatomy, and comparatively little technical training. 

 And not less remarkable than the improvement in cutting 

 sections is the improvement in staining them. Again, the 

 investigator can delegate to an assistant technical work 

 which used to consume the greater part of his own time ; in 

 this work, however, there is hardly a limit to the develop- 

 ment, by practice, of the attendant's skill. Years of train- 

 ing are needed to make him master of some of the more 

 complicated methods of colouring sections of nerve-tissue. 

 Closely associated with improvements in methods of 

 manipulation and observation is the increased control which 

 the observer has acquired over the conditions in which his 

 observations are made. He can vary the temperature from 

 the point at which hydrogen becomes a solid body, within 

 16 or 17 degrees centigrade of absolute zero (below which 

 there is no greater cold, for molecular motion ceases 

 altogether) to the heat of the electric arc in which the most 

 refractory metal passes into the gaseous state. In regard to 

 temperature, therefore, it may almost be said that his 

 experiments may range from the lowest to the highest 

 possible limits. Over pressure, electric tension, light, his 

 control is almost equally extensive. He is no longer com- 

 pelled, like his predecessors in the field, to conjecture that, if 

 it were possible to make an observation under certain con- 

 ditions, the results observed would be thus or thus. No 

 sooner does his argument lead him to infer a certain result 

 than he enters his laboratory, and, having arranged the 

 conditions, brings his hypothesis to the bar of experience. 

 Nay, not only can he command almost every combination of 

 conditions, but he can press into his sen-ice almost every 

 substance which can exist. Compounds which have never 

 been found in nature and have never been formed by art are 

 as much at the chemist's disposal, when he wants them, as 



