37 
once they started they spread to other localities. We hardly knew how 
these diseases go from one place to another. It was difficult to believe 
_they were carried simply by personal communication or by means of the 
wind, and it is almost impossible to believe they crept along the earth’s 
surface, for they usually travelled in one direction and did not extend 
ina circle. But if we did not know how they were produced and pro- 
pagated, we knew that uncleanliness, bad ventilation, and bad drain- 
age, were conditions favourable to such results, and, consequently, that 
cleanliness, good ventilation, and good drainage were the best pre- 
_-  ventitives. When such diseases make their appearance we resort to 
] chemical means to destroy them. And there were many chemical 
compounds which had a most powerful effect in destroying them. 
These compounds acted either by arresting decomposition or by de- 
composing organic matters. When the locality was known where the 
2 disease was, resort was had to such chemicals as were of a less 
volatile nature or were not volatile at all. If, however, the seat of the 
4 disease was not known, but where the rooms were infected with poison- 
_ ous matter, then they applied volatile chemical compounds. The best 
of these were chlorine. This was a volatile gas which was able to fill 
the whole room and enter into every crevice wherever gas or air could 
penetrate. He had just been allowing chlorine to act on some violets, 
which were bleached. This bleaching wasa similar process to what 
was taking place when chlorine attacked organic matters, especially 
such as produced disease. Chlorine had a most powerful affinity for 
hydrogen, and nearly all organic matter was composed of carbon, 
hydrogen, and oxygen in different proportions. The abstraction of the 
hydrogen from the organic compound altered its character so much 
that it lost many of its original properties and got new ones. Thus, 
when a poisonous matter was attacked by chlorine and some of the hy- 
drogen removed, the remaining compound was new, and, as was found, 
an inert one. Chlorine was a most powerful disinfectant, but its affinity 
was notsimply for poisonous matter, but for anyorganic matter. Nobody 
could breathe chlorine, and so the purifying could only last for a short 
time. When the chlorine was removed and the air admitted as before, 
then the room might be entered. Powerful though chlorine was, it 
was not the most practical remedy, because fresh poisonous matter 
might enter when it had been removed. Now, chemical elements 
were usually classified, those of the same class having similar proper- 
ties. Of the same class as chlorine were iodine and bromine. He 
had tried the effects of iodine on smells, and he had found it to destroy 
them ; and, as it could be left in a room, it was a safer and better dis- 
 infectant than chlorine. It would remain like a watch-dog, waiting 
for the enemy, and, as soon as it appeared, destroy it. This was an 
