162 ON ANAESTHETICS 
been brought forward in an exceedingly striking manner by Monsieur Paul Bert. 
Experimenting in a manner essentially similar to that adopted by Dr. Snow, 
diffusing various quantities of the anaesthetic in air contained in a vessel of 
known capacity, so large as to avoid all chance of asphyxial complications, 
he has arrived at remarkable conclusions. So far as I can gather from the 
very brief account which he has published (Comptes Rendus, Nov. 14, 1881), 
he concludes that there is a certain percentage below which chloroform fails 
to produce any anaesthetic effect, however long it continues in operation ; 
that there is another percentage, at or above which it proves mortal (‘]’animal 
finit par mourir’) ; and that between these two definite proportions there is 
a ‘workable zone’ (‘zone maniable’) which produces anaesthesia, but does 
not kill; and when the quantity of chloroform is such as to be about the middle 
of this zone mantable, the animal is rapidly anaesthetized (‘il est tres rapidement 
anesthésié ’), and yet may be safely left for an indefinite time in the same atmos- 
phere (the experiment was sometimes continued as long as two hours), ‘ sans 
qu’on ait a s’occuper ni a s’inquiéter de lui.’ Curiously enough, the smallest 
mortal proportion was just double the smallest anaesthetic quantity ; and this 
held for all kinds of animals, although the absolute amount differed for different 
species. And the same law obtained with ether, bichloride of methylene, and 
other anaesthetics which were made the subjects of experiment. 
These views are so startling that, if it were not for M. Paul Bert’s high 
scientific reputation, we should be disposed to disregard them altogether. There 
can, however, be no doubt of their great importance if correct, and I have 
endeavoured to test their accuracy by some experiments similar to his with one 
of the species which he employed, viz. the mouse, and though my experience 
is very limited, being confined to results obtained from a single family of white 
mice, yet it seems deserving of mention. 
There were four young ones just ready to leave their dam, and I found, 
as Dr. Snow had done, that the young animals of this species were more tolerant 
of chloroform than the adult. The creature was in every case introduced into 
a large glass jar of known capacity, containing air through which a certain 
percentage by weight of chloroform had been thoroughly diffused, assuming 
one hundred cubic inches of air to weigh thirty-one grains, which for practical 
purposes was sufficiently accurate. 
Seven per cent.,? which is the middle of Paul Bert’s zone maniable for the 
mouse, proved much too strong for my animals, causing complete arrest of 
the breathing in one of the young ones in a quarter of an hour, though it recovered 
on being at once withdrawn from the chamber. An adult, the father of the 
' It is to be borne in mind that the percentage by weight is a totally different figure from the per- 
centage by volume used by Dr. Snow and adopted in Part I of this article, the vapour of chloroform 
being 4-2 times heavier than air. 
