460 | Analyses of Books. “6 fJome, 
he draws from his theory, or even to form a very precise concep- 
tion of the theory itself. His reasoning is mductive, and I 
shall endeavour to lay the different steps of the induction before 
my readers. 
1. When electricity passes with facility through any body, 
there is no perceptible heat evolved ; but heat is always produced 
when the electricity passes with a certain degree of difficulty, 
provided it does pass. And the more difficult the passage is, 
the greater is the degree of heat which appears. If we take a 
wire of a given diameter, and cause an electrical shock to pass 
through it, no heat will be produced; but by diminishing the 
diameter of the wire continually, and still transmitting the same 
quantity of electricity through it, we shall find that it will become 
hot, then red-hot, and that it will finally be dissipated in fumes. 
Now the difficulty of the passage of the electricity obviously 
mereases in proportion as the diameter of the wire diminishes. 
2. The better a conductor a metal is, the more difficult it is to 
fuse it by electricity. Thus copper, which conducts electricity 
better than iron, is much more difficult of fusion by electricity 
than iron; so great is the difference that no electrical shock 
that we can produce is capable of fusing more than a very small 
portion of copper wire; while the same shock is capable of fus- 
ing a length of iron wire of the same diameter, amounting to six 
or eight feet. The reason why zinc, lead, tin, &c. are not so 
easily fused by electricity as by heat is obviously the goodness of 
these metals as conductors, which prevents the requisite degree 
of heat for fusing them from being evolved. 
3. When a galvanic current is made to pass through a tube 
filled with water, we shall find that the greatest quantity of heat 
is evolved in the middle of the water, and a thermometer placed 
at the negative extremity acquires the least heat. This was 
proved by a set of experiments made on purpose by (Ersted, and 
by another set by M. Buntzen. In Professor CErsted’s experi- 
ments, the water was contained in a tube composed of sealing- 
wax ; in those of M. Buntzen the water was coniined in a glass 
tube. The reason of the different quantities of heat which 
appear in different parts of the water seems to be this; that in the 
middle of the water no gas whatever is evolved, while at the 
negative end there is an evolution of a greater bulk of gas than 
at the positive end. This gas must deprive the water near it of 
a portion of its heat. 
4. Thus it appears that heat is evolved when electricity is 
transmitted by contact; a fact which seems inconsistent with 
the hypothesis of those who make the heat produced by electri- 
city to depend upon a mechanical vibration; for the vibration 
cannot in such a case be great; and we know how difficult or 
impossible it is to produce heat by the friction of fluids against 
each other. The simultaneous disengagement of gas and heat 
does not seem to accord with the theory of heat at present 
