422 Scientific Intelligence. 
be put for total differential coefficients of V, with respect to 
dV dV 
Y, Zz, &C., saeadt a fast’ Oz, &c.; the same result that would 
be obtained by regarding V at first asa function of only y, 2, 
&c., subject to all the variation of form with respect to these 
variables, which is due to any variation of form that may have 
been attributed to it, with respect to z, y, z, &c., and which is 
due to any variation of form that may be assumed for 2, consider- 
ed as a function of y, 2, &c. y 
SCIENTIFIC INTELLIGENCE. 
I. Cuemistry AND Puysics. 
The substance in question is not wholly a sublimate, being formed 
partly by the condensation of ammoniacal salts and partly by the direct 
action of the gaseous products of combustion on the iron; when the 
fire has been frequently rekindled, there is more or less mixture of car- 
bonaceous matter. 
The action of damp air on this substance when cold, increases the 
corrosion of the pipe, as demonstrated in the report above mentioned. 
This however is not the sole cause, for large flakes may easily be sep- 
arated from pipes which have not been exposed to damp air, and these 
will always be found to contain much iron. 
here is another substance found in the pipes; this is formed of the 
ashes carried up by the draught, and is more or less incrusted and mix- 
ed with the condensed products of combustion. 
Soon after the report of the committee of the Franklin Institute was 
drawn . I found on 
tube, that the glass was corroded. AsT was engaged in determining 
the presence of another substanee, | did not pay much attention to this 
indication of the presence of fluorine. . 
have more recently tried the usual test for fluorine, and find tha 
with a small quantity of the sublimate, the glass is always eo 
even a few grains will suffice to give marks which are visible w 
glass is dimmed by breathing on it.* This reaction uniformly takes 
* If the leaden yessel in which this experiment has been trieds is alitiet 
stand for some time, a large quantity of transparent and perfect octahedrons sr a. 
and ammonia alum will be found. These. may be readily freed from the — 
residue by agitation with a small quantity of water. The crystals however 800 
becume opake and brown when exposed. 
