found that transparent bodies like glass might be very opaque to invisible radiation. Thus, as we all know, a glass screen will keep off the heat of a fire, while if we wish to protect ourselves from the sun, the glass screen will be useless. On the other hand, rock salt freely transmitted invisible radiation. But nothing had been done on the subject of gaseous absorption, when Tyndall attacked this very difficult problem. Some of his results are shown in the accompanying table. The absorption of the ordinary non-condensable, or rather not easily condensable, gases—for we must not talk of non-condensable gases now, least of all in this place—the absorption of these gases is very small; but when we pass to the more compound gases, such as nitric oxide, we find the absorption much greater, and in the case of olefiant gas we see that the absorbing power is as much as 6,000 times that of the ordinary gases.
| Relative Absorption at 1 inch Pressure. | |
| Air | 1 |
| Oxygen | 1 |
| Nitrogen | 1 |
| Hydrogen | 1 |
| Carbonic acid | 972 |
| Nitric oxide | 1,090 |
| Ammonia | 5,460 |
| Olefiant gas | 6,030 |
There is one substance as to which there has been a great diversity of opinion—aqueous vapor. Tyndall found that aqueous vapor exercises a strong power of absorption—strong relatively to that of the air in which it is contained. This is of course a question of great importance, especially in relation to meteorology. Tyndall's conclusions were vehemently contested by many of the authorities of the time, among whom was Magnus, the celebrated physicist of Berlin. With a view to this lecture I have gone somewhat carefully into this question, and I have been greatly impressed by the care and skill showed by Tyndall, even in his earlier experiments upon this subject. He was at once sanguine and skeptical—a combination necessary for success in any branch of science. The experimentalist who is not skeptical will be led away on a false tack and accept conclusions which he would find it necessary to reject were he to pursue the matter further; if not sanguine, he will be discouraged altogether by the difficulties encountered in his earlier efforts, and so arrive at no conclusion at all. One criticism, however, may be made. Tyndall did not at first describe with sufficient detail the method and the precautions which he used. There was a want of that precise information necessary to allow another to follow in his steps. Perhaps this may have been due to his literary instinct.
