SCIENTIFIC CULTURE. 515 



thus find that some of the conditions are merely accidental circum- 

 stances, having no necessary connection with the phenomenon, while 

 others are its invariable antecedent. Having now discovered the 

 true relations of the phenomenon we are studying, a happy guess, 

 suggested probably by analogy, furnishes us with a clew to the real 

 causes on which it depends. We next test our guess by further ex- 

 periments. If our hypothesis is true, this or that must follow ; and, 

 if in all points the theory holds, we have discovered the law of which 

 Ave are in search. If, however, these necessary inferences are not real- 

 ized, then we must abandon our hypothesis, make another guess, and 

 test that in its turn. Let me illustrate by two well-known examples : 

 The, of old, universally accepted principle that all living organ- 

 isms are propagated by seeds or germs {omnia ex ovo) has been seri- 

 ously questioned by a modern school of naturalists. Various observ- 

 ers have maintained that there were conditions under which the lower 

 forms of organic life were developed independently of all such ac- 

 cessories, but other, and equally competent, naturalists who have 

 attempted to investigate the subject, have obtained conflicting re- 

 sults. Thus it was observed that certain low forms of life were 

 quite constantly develo})ed in beef-jnice that had been carefully pre- 

 pared and hermetically sealed in glass flasks, even after these flasks 

 had been exposed for a long time to the temperature of boiling 

 water. " Here," proclaims the new school, " is unmistakable evi- 

 dence of spontaneous generation ; for, if j)ast experience is any guide, 

 all germs must have been killed by the boiling water." "No," answer 

 the more cautious naturalists, " you have not yet proved your jjoint. 

 You have no right to assume that all germs are killed at this temj^era- 

 ture." Tlie experiments, therefore, were repeated under various con- 

 ditions and at different temperatures, but with unsatisfactory results, 

 until Pasteur, a distinguished French physicist, devised a very simple 

 mode of testing the question. He reasoned thus : " If, as is generally 

 believed, the presence of invisible spores in the air is an essential 

 condition of the development of these lower growths, then their 

 production must bear some proportion to the abundance of these 

 spores. Near the habitations of animals and plants, where the 

 spores are known to be in abundance, the development would be 

 naturally at a maximum, and we should expect that the growth would 

 diminish in proportion as the microscope indicated that the spoi'es 

 diminished in the atmosphere." Accordingly, Pasteur selected a re- 

 gion in the Jura Mountains suitable for his purpose, and repeated the 

 well-known experiment with beef-juice, first at the inn of a town at 

 the foot of the mountain^, and then at various elevations up to the 

 bare rocks which covered the top of the ridge, a height of some 8,000 

 feet. At each point he sealed up beef-juice in a large number of 

 flasks and watched the result. He found that while in the town the 

 animalcules were developed in almost all the flasks, they appeared 



