48 MEMOIR OF BARON HALLER. 
transparent. "We see not the wind. Too small and 
feeble to produce any effect upon the surrounding 
fluid, the heart has appeared motionless, as it pre- 
viously had appeared to be wanting. This con- 
sideration should anticipate the conclusion we are 
prone to draw, that an animal lives, or does not 
live, or that it begins to live at this or that moment 
which we choose to fix : we recognize life only by 
motion, and motion is apparent only by a certain 
size and opacity. 
But whence this opacity, and by what shades 
do colours appear ? There is hut one step between 
mucous transparency and whiteness : a little more 
liquid confers transparency on white bodies, and a 
little less deprives them of it. Paper is white, and 
so is pounded glass, yet both become transparent 
when soaked in water or oil : remove these liquids, 
and they again become white. Even the fat of 
living animals is transparent ; a slight dissipation 
of its fluid parts, and its cooling by air, make it 
white. 
White then is the first colour of the animal, as 
transparency is its first condition. This is true of 
all the quadrupeds upon whom I have made expe- 
riments, and these have been very numerous ; the 
same is true respecting birds. The colours are pro- 
duced by the power of the heart dilating the ves- 
sels, and so allowing them to transmit the coloured 
particles, which, according to the principles pointed 
out by Newton, are always larger than diaphanous 
particles. In the chick we find occurring the yellow, 
