4*4 
Mr. de Luc 
steam, or its quantity in a given space ; the observation of 
the temperature is required : as this, by previous experiments 
in order to determine the quantities of steam correspondent 
to its different maxima according to the temperature, will 
afford a co-efficient for the ratio observed. 
15. But are we permitted to consider the variations of the 
hygroscope as proportional to those of moisture in the me- 
dium ? This, according to the above determinations, would 
be the case, if the hygroscopic substance of the instrument 
lengthened in proportion to the quantity of water that it may 
retain in the medium. But the cause of the expansion of those 
substances by water, and the capacity of their pores at diffe- 
rent periods of moisture, are too complicated for answering 
that question d priori ; and by experience, the great differ- 
ences observed in the marches of many of those instruments 
made of different substances, prevents us from assigning that 
property to any of them, till some particular experiment 
comes to help us in that respect. However, that circum- 
stance affects only the practical part of hygrometry, and is 
foreign to the fundamental principles of that science, which 
I have now exposed as they appear to me from its general 
phaenomena. 
I have indicated, in my last paper, two means which I had 
formerly imagined for obtaining that desirable and still want- 
ing correspondence between the march of a determined hy- 
groscope, and that of moisture in the medium. One of those 
means was, to observe, at the same time, the variations in 
weight and length of the same substance, in order to com- 
pare the quantities of water which it retains, with their effects 
on its length. I have executed that experiment ; but its re- 
