52 PHYSIOLOGY OF THE EGGS 
suceceding year, I exposed them to a still greater 
degree of cold. I prepared a mixture of rock salt 
and nitrate of ammonia, and reduced the thermo- 
meter to twenty-two degrees below zero, which was 
twenty-three degrees lower than the cold of 1709. 
They suffered nothing from this rigorous treatment, 
as they were hatched in due season. 
«From these combined facts we mist conclude, 
that cold is less prejudicial to germs and eggs, than 
to animaleula and insects. In general, it is found 
that germs can survive the cold of two degrees below 
zero ; while it is known that some animalcula die at 
the freezing point, and others at about twenty de- 
grees. ‘The eggsof various insects are productive after 
being exposed to a temperature of twenty-two de- 
grees below zero, while insects themselves die at 
sixteen and fourteen degrees. This I have proved 
in the eggs of the Silkworm Moth, and of the Elm 
Butterfly ; and although I ascertained that some 
insects can stand a great degree of cold, I have 
invariably found it to be in a much less ratio than 
what can be resisted by their eggs. What can be 
the cause of this great difference? Insects killed 
at sixteen and fourteen degrees, are so completely 
penetrated and frozen, that their members do not 
yield to the pressure of the finger, and even under 
the knife they appear perfect ice. This is not the 
case with eggs; for the contents of the shell, or - 
crust, remain quite fluid under the influence of the 
