diseases
literally plant their seeds, grow, and shake abroad new germs, which,
meeting in the human body their proper food and temperature, finally
take possession of whole populations. There is nothing to my
knowledge in pure chemistry which resembles the power of propagation
and self-multiplication possessed by the matter which produces
epidemic disease. If you sow wheat you do not get barley; if you sow
small-pox you do not get scarlet-fever, but small-pox indefinitely
multiplied, and nothing else. The matter of each contagious disease
reproduces itself as rigidly as if it were (as Miss Nightingale puts
it) dog or cat.

Parasitic Diseases of Silkworms. Pasteur's Researches.

It is admitted on all hands that some diseases are the product of
parasitic growth. Both in man and in lower creatures, the existence
of such diseases has been demonstrated. I am enabled to lay before
you an account of an epidemic of this kind, thoroughly investigated
and successfully combated by M. Pasteur. For fifteen years a plague
had raged among the silkworms of France. They had sickened and died
in multitudes, while those that succeeded in spinning their cocoons
furnished only a fraction of the normal quantity of silk. In 1853 the
silk culture of France produced a revenue of one hundred and thirty
millions of francs. During the twenty previous years the revenue had
doubled itself, and no doubt was entertained as to its further
augmentation. The weight of the cocoons produced in 1853 was
26,000,000 kilogrammes; in 1865 it had fallen to 4,000,000, the fall
entailing, in a single year, a loss of 100,000,000 francs.

The country chiefly smitten by this calamity happened to be that of
the celebrated chemist Dumas, now perpetual secretary of the French
Academy of Sciences. He turned to his friend, colleague, and pupil,
Pasteur, and besought him, with an earnestness which the circumstances
rendered almost personal, to undertake the investigation of the
malady. Pasteur at this time had never seen a silkworm, and he urged
his inexperience in reply to his friend. But Dumas knew too well the
qualities needed for such an enquiry to accept Pasteur's reason for
declining it. 'Je mets,' said he, 'un prix extreme a voir votre
attention fixee sur la question qui interesse mon pauvre pays; la
misere surpasse tout ce que vous pouvez imaginer.' Pamphlets about the
plague had been showered upon the public, the monotony of waste paper
being b