Tyndall also demonstrated that polarity, or the power of a substance to attract one pole of a magnetic needle and repel the other, was also a property of diamagnetic substances, with the difference that, if placed under the same magnetizing influence, a bar of diamagnetic substance would show north polarity at that end which in a bar of iron or other magnetic substance would be a south pole. It was also shown that the attractive force of magnetism is infinitely greater than diamagnetism, the magnetism of iron exceeding the diamagnetism of bismuth two and a half million times.

In 1859 Tyndall began his researches in radiant heat, a subject of great interest, not only to scientists but to all who are desirous of understanding the relations which exist between the forces of nature and the laws of life.

The power of the atmosphere to absorb the heat of the sun was then attracting attention, as it is a questions bearing directly upon human interests, as well as being a valuable subject for scientific inquiry.

The Italian physicist, Melloni, had made some very important researches in radiant heat, and had given special study to its passage through difference substances.

A body which allows heat to pass through it is said to have the property of diathermancy, just as a body which allows light to pass through it is said to have the property of transparency. And Melloni, by a series of interesting experiments, established several laws in regard to the diathermancy of different substances.

Rock salt was found to possess great diathermancy, as it allowed nearly all the heat to pass through; glass, on the contrary, which was as transparent as rock salt, was found to have little power of transmitting heat; ice and alum, equally transparent, have slight diathermancy, while clear and smoky quartz, one as transparent as glass, and the other nearly opaque, alike transmit considerable heat.

Tyndall's experiments related chiefly to the diathermancy of gases, and proved that the heat in gases and vapors was absorbed and radiated with as great differences as those which marked its passage through liquids and solids, and that it was governed by certain laws which played an important part in the distribution of heat over the world.

He found that dry air permitted heat to pass freely, but that watery vapor was possessed of great power for absorbing the heat, and this conclusion was made the basis of a most interesting hypothesis in regard to the distribution of heat over the globe.

Countries distinguished by a moist climate, like England or Ireland, were thus particularly favored, as the watery vapor, which Tyndall likened to a blanket, absorbed the heat which would otherwise have passed off by radiation from the earth, and kept a sufficient warmth to protect vegetation, just as clothing protects the human frame; and Tyndall said that if this watery vapor were removed form the air for a single summer night, the sun would rise the next day upon an island held fast in the iron grip of frost, with every plant and flower dead.

The absence of watery vapor in the atmosphere would, in like manner, account for the terrible cold of dry climates, such as Central Asia, and the nights of the Sahara desert. This theory was of special importance to geology, as it explained the origin of the glacial era; for as the earth was passing through its cooling period, the oceans, as is now the case, would naturally be warmer than the land, owing to the presence of watery vapor over their surface which served as a blanket to keep in the heat; the dry air over the land would permit the heat to pass off rapidly into space, on the contrary; and thus the rapid cooling of the land turned the mountains into receivers of the condensing vapors, which formed into the great glaciers which once covered the earth.

Another very interesting study of radiant heat, made by Professor Tyndall, related to the separation of the invisible from the visible waves or rays of light.

The fact that the light of the sun as reflected from the moon has very little heating power in proportion to its illuminating effect, had suggested to Melloni the idea of a set of experiments which resulted in the separation of heat from light on a smaller scale, and Tyndall made some successful experiments showing that the reverse was also true. In these experiments he separated the visible from the invisible rays of the sun, the lime light, and electric light, allowing the dark rays, which have the principal heating power, to pass through the intercepting medium that he used, while at the same time not a ray of light was received. With these dark rays he produced fire, melted metals, and obtained the different-colored rays of light, thus proving that the invisible rays of the sun may carry on the mightiest operations of nature, just as surely as the flower may give forth its fragrance in the darkness.

In this connection Tyndall invented a respirator for the use of firemen. This instrument, which consisted of layers of moist wool, dry wool, charcoal fragments, and caustic lime, enclosed in a wire gauze, was found to be a great protection to firemen who were unable to carry on their duties in consequence of the smoke from the burning building. The respirator effectually destroyed the bad effects of the smoke, and allowed the firemen to breathe in a room filled with the densest smoke without discomfort.

In his researches on light Tyndall also gave his attention to sound, and its relation to heat. Seamen had often been puzzled by the fact that the signals used during fogs often failed to convey the warnings in fine weather, and that the guns, gongs, and powerful whistles heard miles away during the rain could not be distinguished sometimes at short distances when the sun was shining. Tyndall suggested that this was due to the presence of invisible clouds which formed a barrier to the waves of sound, just as a dark cloud shuts out the sunshine; and, pursuing this subject later on he found that certain vapors and gases possessed the power of conveying sound in the same order as their absorption of radiant heat.

Some of the experiments leading to this conclusion related to the conversion of light into sound. Starting from the fact that thin disks of metal would produce musical sounds when struck by an intermittent beam of light, Professor Tyndall carried on a number of experiments which proved to his satisfaction that such a beam of light striking a highly absorbent vapor would even produce a more intense sound than that produced by a solid. The test experiment consisted of an arrangement by which the light struck the vapor only at intervals, the sounds being caused by the alternate expansion and contraction of the vapor, it being found that vapors and gases which allowed the heat to pass through them would produce no sounds whatever. Chloride of methyl was found to give forth sounds which, when conveyed to the ear by a rubber tube, resembled the peal of an organ in intensity.

In his pursuit of science Tyndall has added the advantages of travel, and his study on the glaciers of the Alps and the Falls of Niagara have an especial interest from the fact that they were carried on in the midst of dangers that might well have deterred a less devoted seeker after truth.

Professor Tyndall possesses a remarkable faculty for making his subjects of study understood by the unscientific mind, and his lectures in England and America have done much to make the study of science and its high objects popular, while his uncompromising love of truth, and his unimpeachable honesty in its pursuit have won him distinction from his fellow-laborers in the fields of knowledge.