1817.J and a new Hygrometer. 319 
are their tendency to change in sensibility, the necessity of some 
manipulation, or their not being comparative. oie 
To overcome all these objections would be no easy matter. I 
will not venture to assert that I have done so; but the instrument to 
be described is more simple and sensible than any I have yet seen. 
During a variety of experiments made some years ago in the 
pursuit of hygrometry, I found that slips of animal bladders were 
very susceptible of changes in humidity. ‘The idea naturally sug- 
gested itself of trying whether the internal capacity of the whole 
bladder did not likewise change with the degree of moisture to 
which it was exposed. I put this to the test of experiment with the 
gall bladder of a sheep filled with mercury, and I obtained an un- 
looked-for delicacy and precision in the result. I found that, on 
its being repeatedly immersed in water of the same temperature, the 
mercury always fell to the same point, and that it likewise always 
rose to the same height when included in air exposed to the ab- 
sorbent power of sulphuric acid of the same specific gravity. This 
range is considerable, being at least three times as much as would 
be caused in a thermometer between the freezing and boiling 
points, in which the bulb and tube were in the same ratio to each 
other as the bladder and tube of the hygrometer. 
This seems a general property of bladders. 1 have tried those of 
many animals, but give the preference to the urinary bladder of a 
rat,* on account of its small size, and its extreme sensibility. 
In order to construct this instrument, procure the urinary bladder 
of a rat, or other small animal, wash it well in cold water, and 
turn it inside out. By means of a file, notch a small part of the 
extremity of a thermometer tube, and insert it a little way into the 
orifice of the bladder; then tie it on firmly with a silk thread, and 
make a puncture with a fine needle immediately below the fasten- 
ing, in order to allow the air to escape as the mercury enters, The 
most convenient method of filling it is to put rather more mercury 
than what is necessary into the gall bladder of a lamb; this bladder, 
previously moistened, is then attached in a temporary way to the 
other extremity of the thermometer tube, and by gently inclining 
it the mercury will run down, and displace the air in the rat’s 
bladder, and ultimately escape through the puncture. When this 
takes place, the hole must be secured, by extending the tying of 
the silk thread over it, and making it fast. It is necessary to keep 
the rat’s bladder wet during the operation, and not to allow too 
great a column of mercury to press on it, otherwise it will be apt 
to burst when in this state. 
The mercury is new to be adjusted, until it stands at a conve- 
* It may perhaps lead to some useful result fo state that J have found the rats 
in London to be very subject to urinary calculi; which f did not find to be the 
casein other towns. ‘The bladders of some are entirely filled with a white spongy 
matter; others withred gritty sand, Ihave not yet submitted any of these calculi 
to anal ysis. 
