500 THE LIFE-WORK OF A CHEMIST. 



I have already pointed out bow a pure cultivation of a microbe can be 

 effected. Just as tbe production of pure alcohol depends on the pres- 

 ence of the pure yeast, so special diseases are dependent on the presence 

 of certain definite organisms which can be artificially cultivated, and 

 which give rise to the special malady. Can we now by any system of 

 artificial cultivation so modify or weaken the virus of a given microbe 

 as to render it possible to inoculate a modified virus which, whilst it is 

 without danger to life, is still capable of acting as a preventive to fur- 

 ther attack? This is the question which Pasteur set himself to solve, 

 nor was the task by any means an apparently hopeless one. He had 

 not only the case of Jennerian vaccination before him, but also the well- 

 known modifications which cultivation can bring about in plants. The 

 first instance in which Pasteur succeeded in effecting tbis weakening 

 of the poison was in that of a fatal disease to which poultry in France 

 are very liable, called chicken cholera. Like many other maladies, this 

 is caused by the presence of a micro-organism found in the blood and 

 tissues of the stricken fowl. One drop of this blood brought under the 

 skin of a healthy chicken kills it, and the same microbe is found through- 

 out its body. And if a pure culture of these microbes be made, that 

 culture — even after a series of generations — is as deadly a poison as the 

 original blood. Now comes the discovery. If these cultures be kept 

 at a suitable temperature for some weeks exposed to pure air, and the 

 poisonous properties tested from time to time, the poison is found grad- 

 ually to become less powerful, so that after the lapse of two months a 

 dose which had formerly proved fatal now does not disturb in the 

 slightest the apparent health of the fowl. But now let us inoculate a 

 chicken with this weakened virus. It suffers a slight illness, but soon 

 recovers. Next let us give it a dose of tbe undiluted poison, and, as a 

 control, let us try the action of tbe same on an unprotected bird. Wbat 

 is the result i? Why, that the first chicken renuiins unaffected, whilst 

 the second bird dies. Tbe inoculation has rendered it exempt from the 

 disease, and this has been proved by Pasteur to be true in thousands 

 ot cases, so that wbereas the death-rate in certain districts amongst 

 fowls before tbe adoption of Pasteur's inoculation method was 10 per 

 cent., after its general adoption it has diminished to less than 1 per 

 cent. 



We can scarcely value too highly tbis discovery, for it proves that 

 the poisonous nature of the microbe is not unalterable, but that it can be 

 artificially modified and reduced, and thus an explanation is given of 

 the fact that in an epidemic the virus may either be preserved or be- 

 come exhausted according to tbe conditions to which it is subjected. 

 We have here to do with a case similar to that of Jenner's vaccine, 

 except that here the relation between the weak and tbe strong i)oison 

 has become known to us, whilst in Jenner's case it has lain concealed. 

 Tbis then is tbe first triumph of experimental inquiry into tbe cause 

 and prevention of microbic disease, and tbis method of attenuation is 



