general considerations, however, it appears that the material basis of this machine is to be found in the air medium which surrounds the two bodies. Experiment shows, indeed, that if this intervening material medium of air be even partially withdrawn or removed, the transmission is immensely reduced in amount. In fact, this latter phenomenon is largely taken advantage of in the so-called vacuum flasks or other devices to maintain bodies at a temperature either above or below that of the external surrounding bodies. The device adopted is, simply, as far as practicable to withdraw all material connection between the body which it is desired to isolate thermally and its surroundings. But it is clearly impossible to isolate completely any terrestrial body in this way. There must be some material connection remaining. As already pointed out (§ 5), we have no experimental experience of really separate bodies or of an absolute vacuum. It is to be noted that any vacuous space which we can experimentally arrange does not even approximately reproduce the conditions of true separation prevailing in interplanetary space. Any arrangement of separate bodies which might thus be contrived is necessarily entirely surrounded or enclosed by terrestrial material which, in virtue of its stressed condition, constitutes an energy machine of the same nature as those already described (§ 21). Even although the air could be absolutely exhausted from a vessel, it is still quite impossible to enclose any body permanently within that vessel without some material connection between the body and the enclosing walls. If for example, as shown in Fig. 6, CC represents a spherical vessel, completely exhausted, and having two bodies, A and B at different temperatures, in its interior, it is obvious that if these bodies are to maintain continuously their relative positions of separation, each must be united by some material connection to the containing vessel. But when such a connection is made, say as shown at D and E (Fig. 7), it is clear that A and B are no longer separate bodies in the fullest sense of the word, but are now in direct communication with one another through the supports at D and E and the enclosing sides of the vessel CC. The practicable conditions are thus far from those of separate bodies in a complete vacuum. It would seem, indeed, to be beyond human experimental contrivance to reproduce such conditions in their entirety. So far as these conditions can be achieved, however, and judging solely by the experimental results already attained with respect to the effect of exhaustion on radiation, it may be quite justly averred that, if the conditions portrayed in Fig. 6 could be realised, no transmission of energy would take place between two bodies, such as A and B, completely isolated from one another in a vacuous space. It appears, in fact, to be a quite reasonable and logical deduction from the experimental evidence that the energy operation of transmission of