VACCINES 415 



bathing tubs which contain the droppings of hens, pigeons, and dogs. 

 Here a true digestion of the hide is carried on by bacteria. Wood 

 has isolated 90 species of bacteria from such a tube, no one of which 

 possessed the power of bringing about the desired change, but all 

 acting conjointly gave the desired product. The heated hides are 

 next placed in a tan pit or in bark liquor. Numerous bacteria 

 yeast and molds occur in the bark liquor and play a part in the 

 finishing of the product. 



Vaccines. A vaccine is a killed or weakened (attenuated) sus- 

 pension of organisms to be inoculated into the body for the purpose 

 of causing the development of an active immunity. 



The vaccine is usually prepared from a fresh twenty-four-hour 

 growth of the microorganism on agar. The surface growth only is 

 taken, thus avoiding secondary metabolic products which may be 

 formed. The cultures are usually killed by exposure to heat at 

 from 53 to 60 C. for one hour. High heat, while certain to kill 

 the virus, is undesirable for the reason that it coagulates the protein 

 substances in the bacterial cell and otherwise alters its chemical 

 structure. The closer the vaccine approaches the virus the better 

 the result and higher the resulting immunity. For this reason many 

 workers prefer to kill the bacteria with chemicals, carbolic acid, 

 chloroform, or some other suitable germicide. 



The attenuated virus is obtained by passing the microorganism 

 through the body of some animal, as smallpox through the heifer 

 by which its virulence for man is reduced. At other times it is 

 grown under adverse conditions high temperatures, artificial media, 

 or in the presence of antiseptics, after which it becomes less virulent. 

 Drying is used in the case of the virus of rabies. 



In the preparation of antitoxins the bacterial cell or some of its 

 products are injected into a suitable animal, and after sufficient 

 time has elapsed blood is drawn and after appropriate treatment is 

 used for the cure or prevention of disease. 



REFERENCES. 



In addition to the references listed at the end of the several chapters, the fol- 

 lowing have been freely consulted and to these, the students are referied for further 

 information. The date given is that in which the first volume appeared. 



Abstracts of Bacteriology, 1917, vol. i. 



Agricultural Index, 1916, vol. i. 



Biedermann's Centralblatt fur Agrikulturchemie, 1872, Bd. i. 



Botanical Gazette, 1876, vol. i. 



Centralblatt fur Bakteriologie, 1887, Abt. I, Bd. i. 



Centralblatt fur Bakteriologie, 1892, Abt. II, Bd. i. 



Chemical Abstract, 1907, vol. i. 



Experiment Station Record, 1888, vol. i. 



Comptes Rendus Academic des Sciences, 1835, vol. i. 



Experiment Station Bulletins. 



International Catalogue of Scientific Literature, 1911. 



Jahresbericht iiber die Fortschritte der Agrikulturchemie, 1858, Bd. i, 



