BATH CHEMISTS 5 ASSOCIATION. 
571 
levers, would measure accurately a space less than the J„th part of a millimetre; by 
this means the gradual expansion of zinc and copper wires was observed with the 
greatest facility. The vibration of atoms was proved by a very pretty experiment, m 
which some tubes, containing a few of the earthy sulphides, were subjected to actinic 
rays. Each immediately gave its characteristic colour. The production ot heat and 
cold were shown by solution, crystallization, evaporation, expansion, and conti action o 
different substances. Specific heat was then explained and illustrated by examp es, as 
was also the well-known law of Dulong and Petit, and its use to the analytical chemis . 
The author then proceeded to review the properties of heat, and pointed out their great 
resemblance to those of light, as polarization, reflection, radiation, and absorption. y 
means of a bright reflector, the rays of heat, and apparently those of cold, were shown to 
be reflected at the angle of incidence. An application of this property occurs to the 
pharmaceutist, for should he want his boiling-pans to make a decoction quick y, e 
would do very wrong in keeping the exterior of his pans bright and shining, bo muci 
heat would be lost by reflection, that his gas would be wasted; whereas by aHowmg the 
vessel to become coated with smoke or roughened, his decoction would arrive at the boil¬ 
ing-point in the least possible time, and with the least quantity of gas or fire. I he 
radiation of heat most frequently concerns our every-day business. The author explained 
radiation in the words of Professor Tyndall,—that it is the communication from the 
particles of a heated body to the ether, in which these bodies are immersed, all atoms 
being capable of being put into an intense state of vibration, which (vibration) is com¬ 
municated like sound by means of undulations. It was then stated, that when the in¬ 
terstitial ether was at rest it was quiet and neutral, but that when excited, undulating 
waves are formed which do not proceed longitudinally as sound in air, but transversely, 
and whereas only sixteen vibrations of air per second are required to produce a sound, it 
takes many millions of millions of vibrations of the ether per second to produce a single 
rav of heat When the number per second of these vibrations reach 458 millions of 
millions, we see red light, such as would be seen in a glowing fire; when they exceed 
727 millions of millions per second, they become too numerous for the eye to see without 
special arrangements, but we then have all the actinic properties ot the Herschelian 
spectrum. This was shown by the fluorescent effects of a quinine solution which, when 
poured into water and viewed by the magnesium light appeared as a thick dense fluid 
falling through a light one. Many experiments fully explaining the phenomena ot 
radiation were shown, by means of two large hollow; cubes filled with hot 
side beino- covered by different colours ; the coloured sides (red and black) radiated freely, 
while the naked metallic side hardly radiated at all. One of the naked sides, covered 
with a thin coat of varnish, also gave off copious rays. The pharmaceutist at once will 
understand the necessity, therefore, of keeping his crystallizmg-pans steam-pipes in¬ 
fusion-jugs, etc., perfectly bright, or any vessel containing a liquid that he wishes to 
cool very gradually. A bright pewter infusion-jug will take a very much longer time 
to cool than a dirty one. The engineer knows full well that if he does not keep his 
steam-nmes perfectly bright, a great percentage of his steam will be condensed, and he 
will not get the full power out of his locomotive. Glass and pewter radiate much more 
than the metals therefore the chemist knows that the latter are the best material for an 
nfusion-iug ' A^very familiar instance of the difference in radiating powers is known 
to most housekeepers, by the custom of covering up the teapot with a woollen covering 
to prevent its cooling. ‘ The benefit is often imaginary, for if the covering be not very 
loose indeed the tea would be cooled very rapidly, because the woollen material is a 
better radiator than the bare metal. The author proved this, by loosely covering a 
bare metal cube, when the rays impinging upon the reflector of the thermop.le were 
increased slightly, but when the cover was drawn tightly round the cube, the rays were 
evolved in the most copious manner. This explains our packing ice in a flannel during 
the summer, and our pumps in straw in the winter. Gas stoves are another illustration: 
if made of bright metal, the room would take a long time to get warm ; they are there¬ 
fore always blackened.' Heat, like light, passes easily through some solids, but not 
through others. Those substances transmitting heat are termed “ diathermanous, and 
those stopping the passage of the heat-rays “ athermanous. It is very remarkable that 
transparent todies Ire not always diathermanous. Pieces of alum and 
and proved to be nearly athermanous although perf ectly clear and transparent wh,l e a 
piece of rock salt, when blackened, allowed most of the heat-rays to pass, the best 
