_Thermal Transparency at Low Temperatures._--The proposition, once enunciated by Pictet, that at low temperatures all substances have practically the same thermal transparency, and are equally ineffective as non-conductors of heat, is based on erroneous observations. It is true that if the space between the two walls of a double-walled vessel is packed with substances like carbon, magnesia, or silica, liquid air placed in the interior will boil off even more quickly than it will when the space merely contains air at atmospheric pressure; but in such cases it is not so much the carbon, &c., that bring about the transference of heat, as the air contained in their interstices. If this air be pumped out such substances are seen to exert a very considerable influence in stopping the influx of heat, and a vacuum vessel which has the space between its two walls filled with a non-conducting material of this kind preserves a liquid gas even better than one in which that space is simply exhausted of air. In experiments on this point double-walled glass tubes, as nearly identical in shape and size as possible, were mounted in sets of three on a common stem which communicated with an air-pump, so that the degree of exhaustion in each was equal. In two of each three the space between the double walls was filled with the powdered material it was desired to test, the third being left empty and used as the standard. The time required for a certain quantity of liquid air to evaporate from the interior of this empty bulb being called 1, in each of the eight sets of triple tubes, the times required for the same quantity to boil off from the other pairs of tubes were as follows:-- / Charcoal 5 / Lampblack 5 \ Magnesia 2 \ Silica 4 / Graphite 1.3 / Lampblack 4 \ Alumina 3.3 \ Lycopodium 2.5 / Calcium carbonate 2.5 / Barium carbonate 1.3 \ Calcium fluoride 1.25 \ Calcium phosphate 2.7 / Phosphorus (amorphous) 1 / Lead oxide 2 \ Mercuric iodide 1.5 \ Bismuth oxide 6 Other experiments of the same kind made--(a) with similar vacuum vessels, but with the powders replaced by metallic and other septa; and (b) with vacuum vessels having their walls silvered, yielded the following results:-- /(a) Vacuum space empty 1 / Vacuum space empty 1 | Three turns silver | Three turns black paper, | paper, bright < black outside 3 < surface inside 4 | Three turns black paper, | Three turns silver \ black inside 3 | paper, bright \ surfaceoutside 4