Chemistry. — 405 
ture ; which necessitates, that a molecule, or repellent nucleus, shall be 
of equal temperature with those that are in contact with it, And the 
reason of this law is axiomatic; for that all motion is attended with the 
loss or attainment of an equilibrium. All forces of action and reaction, 
attraction and repulsion, develope each other, and are therefore equal. 
“* When two nuclei, the same in kind, but differing in size, are brought: 
in contact, they will presently divide the difference, both attaining the 
mean diameter, and the mean intensity of repulsion ; for it is necessary 
toadmit, that the intensities of the spheres of temperatures vary in- 
versely as the cubes of their diameters. But if they be of different spe- 
cies, (e. g. gold and iron,) one will contract, or expand, more than the 
other. Thus, gold, under the same influences, contracts, or cools, more 
than iron; and this difference expresses what is Mconveniently termed, 
their specific heat ;’ meaning their ‘relative degrees of insuscepti- 
bility.’ It would be more convenient to speak of their susceptibility.” 
~Itis necessary to caution the chemist, against confounding the ‘ nu- 
clei’ of this theory, with the “hard particles” of the Epicurean, or 
Wollastonian, hypothesis: they are not ‘hard, visible, particles,’ but 
mere variable, invisible, limits; in which the repellent, and cohesive, 
forces are developed. 4 
2. On a simple. method of protecting from Lightning, Buildings 
with Metallic Roofs; by Prof. Henry, (from Proceedings of Ameri- 
can Phil. Soc., June 20, 1845.)—On the principle of electrical induc- 
tion, houses thus covered are evidently more liable to be struck than 
those furnished either with shingle or tile. Fortunately, however, they 
admit of very simple means of perfect protection. It is evident, from 
well established principles of electrical action, that if the outside of a 
Were encased entirely in a coating of metal, the most violent dis- 
charge which might fall upon it from the clouds would pass silently to 
the earth without damaging the house, or endangering the inmates. It 
's also evident, that if the house be merely covered with a roof of metal, 
Without projecting chimneys, and this roof were put in metallic connec- 
ion with the ground, the building would be perfectly protected. To 
make a protection, therefore, of this kind, the Professor advises that the 
metallic roof be placed in connection with the ground by means of the 
T'oe Copper gutters which serve to lead the water from the roof to the 
earth. For. this purpose, it is sufficient to ‘solder to the lower end of 
the gutter a riband of sheet copper, two or three inches wide, surround- 
ing it with charcoal, and continuing it out from the house until it ter- 
Minates in moist ground. ‘The upper ends of these gutters are generally 
Soldered to the roof; but if they are not in metallic connection, the two 
Should ‘be joined by a slip of sheet copper. The only part of the house 
tected by this ar rangement will be the chimneys ; and in order ta 
