THE LIFE-WORK OF A CHEMIST. 495 



turns sour. But still more, the organism (j^east) wliicli brings about 

 the alcoholic fermentation will not give rise to the acetous, and vice 

 versa ; so that each peculiar chemical change is brought about by the 

 vital action of a peculiar organism. In its absence the change can not 

 occur ; in its presence only that change can take place. 



Here again we may ask, as Pasteur did, why does beer or wine be- 

 come sour when exposed to ordinary air ? And the answer to- this 

 question was given by him in no uncertain tone in one of the most 

 remarkable and most important of modern experimental researches. 

 Milk and beer which have become sour on standing in the air contain 

 living micro-organisms which did not exist in the original sound fluids. 

 Where did these organisms originate? Are they or their germs con- 

 tained in the air, or are these minute beings formed by a process of 

 spontaneous generation from material not endowed with life ? 



A controversy as to the truth or falsit^^ of the theory of spontaneous 

 generation was waged with spirit on both sides, but in the end Pas- 

 teur came off victorious, for by a series of the most delicate and con- 

 vincing of experiments he proved the existence of micro-organic forms 

 and their spores — or seeds — in the air, and showed that while unpuri- 

 fied air was capable of setting up fermentative changes of various 

 kinds, the same air freed from germs could not give rise to these 

 changes Keep away the special germ which is the incentive to the 

 pathological change and that change can not occur. In the interior 

 of the grape, in the healthy blood, no such organisms, no such germs 

 exist; puncture the grai)e or wound the animal body and the germs 

 floating in the air settle on the grape juice or on the wounded tissue, 

 and the processes of change, whether fermentative or putrefactive, set 

 in with all their attendant symptoms. But crush the grape or wound 

 the animal under conditions which either preclude the presence or de- 

 stroy the life of the floating germ, and again no such change occurs; 

 the grape-juice remains sweet, the wound clean. 



I have said that every peculiar fermentative change is accompanied 

 by the presence of a special ferment. This most important conclusion 

 has only been arrived at as the result of careful experimental inquiry. 

 How was this effected? By the artificial cultivation of these organ- 

 isms. Just as the botanist or gardener picks out from a multitude of 

 wild plants the special one which he wishes to propagate, and planting 

 it in ground fiivorable to its growth, obtains fresh crops of the special 

 plant he has chosen, so the bacteriologist can, by a careful process of 

 selection, obtain what is termed a pure cultivation of any desired organ- 

 ism. Having obtained such a pure cultivation, the next ste[) is to as- 

 certain what are the distinctive properties of that special organism 5 

 what characteristic changes does it bring about in material suitable for 

 its growth. This having been determined, and a foundation for the 

 science having thus been laid, it is not diflicult to apply these principles 

 to practice, and the first applicatioo mai-l^ by Pasteur wa§ to tUe 6tw(iy 

 of tlie clisea§es of b^er mA wine, 



