LEAD IN VARIOUS ARTIFICIAL PRODUCTS. 305 
Hollandaise,) I quoted the experiments, from which it ap- 
peared to me that the copper which analysis may discover 
in animals or plants, is not to be considered as an essential 
element of their constitution. The quantity of this metal, 
though always very small, is variable, being sometimes 
altogether absent. If a cupreous salt penetrates the soil of 
vegetables by means of water, or a similar salt or cupreous 
powder penetrates with the food, or in any manner what- 
ever the body of animals or of man, I consider this pene- 
tration as an accidental one. This is in conformity with 
the views expressed in my treatise on "-Matter considered 
as it exists in Living Beings" in which I distinguished 
three classes of immediate principles — principles essential 
to the existence of beings; principles which, though neces- 
sary, are not essential, so that, if they are absent, others 
may supply their place, finally, accidental principles which, 
without inconvenience, may be absolutely absent. I placed 
lead and copper among the accidental principles of the or- 
ganised beings. 
I will now make some remarks with reference to the 
presence of this metal, deduced from analyses made under a 
circumstance of which I am about to speak. Having put 
some cloth, silk and wool, to be used in the demonstration 
of my course, (delivered last year at the Gobelins,) in water 
rendered alkaline, it was noticed with astonishment that 
the wool became brown in waters containing soda, baryta, 
strontia and lime, which were prepared for my experiments 
with perfectly pure alkali. I soon recognised the action of 
the oxide of lead under the influence of the alkali and sul- 
phur of the wool, by the coloration of this latter body. But 
whence came the oxide which I found in the alkaline waters 
on treating them with sulphuretted hydrogen? It came 
from the flasks in which these waters were kept during 
several months, and it was evident that the glass of these 
flasks was a mixture of glass, properly so-called, and of 
lead-glass, called crystal. In the year 1828, I pointed out 
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