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 
