S 3 6 Mr. Davy's Experiments and Observations 
and 70° Fahrenheit; and the solutions of gelatine that I have 
used, were made by dissolving 120 grains of isinglass in 20 
ounces of water. 
In ascertaining the proportions of tannin in astringent infu- 
sions, great care must be taken to prevent the presence of any 
excess of gelatine; for, when this excess exists, I have found 
that a small portion of the solid compound formed is redissolved, 
and the results of the experiment otherwise affected. It is not 
difficult to discover the precise point of saturation, if the solution 
of isinglass be added only in small quantities at a time, and if 
portions of the clear liquor be passed through a filtre at different 
periods of the process. The properties of these portions will 
indicate the quantities of the solution of gelatine required for 
the completion of the experiment. 
That the composition of any precipitate containing tannin and 
gelatine may be known with a tolerable degree of precision, it 
is necessary that the isinglass employed in the solution, and the 
new compound formed, be brought as nearly as possible to the 
same degree of dryness. For this purpose, I have generally ex- 
posed them, for an equal time, upon the lower plate of a sand- 
bath, which was seldom heated to more than 150°. This method 
I have found much better than that of drying at the temperatures 
of the atmosphere, as the different states of the air, with regard 
to moisture, materially influence the results. 
Mr. Hatchett has noticed, in his excellent Paper on Zoo- 
phytes, &c.* that isinglass is almost wholly composed of gelatine. 
I have found, that 100 grains of good and dry isinglass contain 
rather more than 98 grains of matter soluble in water. So that, 
when the quantity of isinglass, in any solution employed for 
* Phil. Trans, for 1800, page 327. 
