MEANS OF DEFENCE AGAINST MICROBES. 251 



of perfectly pure water may be obtained in the course 

 of an hour. Under the pressure of the taps of the 

 Paris water-supply, the jet of the filtered water is 

 as strong as that of the pipes used for watering our 

 gardens; in fact, it gives out four or five litres a 

 minute under the pressure of two or three atmospheres. 



Preservation of Alimentary Substances. Appert's 

 Protective Process, etc. We have already said that 

 organic substances may be preserved unchanged for 

 an indefinite time, as long as they are protected from 

 the microbes and germs in the air. This was shown 

 by Pasteur's exhaustive experiments. He took urine 

 and blood, and transferred them directly from the 

 animal organs into glass flasks which had been pre- 

 viously sterilized or deprived of all germs. These 

 flasks were hermetically closed and kept for forty-five 

 days. When opened at the end of that time, it was 

 ascertained that the smell and appearance of the 

 liquids were unchanged, that no putrid gas had been 

 developed, and even that some of the oxygen in the 

 flasks had not been absorbed. 



Most of the processes in use, even before this 

 experiment, for the preservation of food substances, 

 are only the practical application of this principle : 

 the exclusion of microbes and germs. 



Appert's process, now so generally used to preserve 

 meat and vegetables, consists in enclosing the sub- 

 stances to be preserved in tins, which are hermetically 

 closed, and heated to a temperature of 110, so as 



