Page:Dictionary of National Biography volume 18.djvu/205

Faraday diamagnetic repulsion. Suspended before either pole of an electro-magnet it was repelled when the force was developed. Suspended as a bar between the two poles, it retreated when the magnet was excited, setting its length at right angles to the line joining the poles. A magnetic bar, similarly suspended, always set its length from pole to pole. The first of these positions Faraday called the 'equatorial' position, the second the 'axial' position. In accordance with his usual habit he pushed his experiments on diamagnetism in all possible directions. He subjected bodies of all kinds to the action of his magnet, and found that no known solid or liquid was insensible to magnetic power when it was developed in sufficient strength. Faraday himself was the first to throw out the hypothesis that the deportment of diamagnetic bodies could be explained by assuming in their case a polarity the reverse of that exhibited by magnetic bodies. This hypothesis, however, was but loosely held, and his own experiments failed to furnish any evidence of its truth. The instruments employed by Faraday in his investigations on diamagnetic polarity lacked the necessary delicacy, and failed to show him a quality and character of this new repellent force, in every respect as certain as ordinary magnetic polarity. But though this fundamental quality of the force he had discovered eluded his experimental devices during the course of the discussion were of surpassing beauty. His experiments and speculations in the deportment of crystals in the magnetic field, a deportment predicted by Poisson, and discovered experimentally by the illustrious geometrician Plucker, are profoundly interesting and instructive. They throw more light than any others on the character of Faraday's mind and culture. He invented new terms to describe and new forces to explain magne-crystallic phenomena. It is marvellous how true his instincts were, even where his speculations were invalid. Through reasonings often confused, he passed to experimental results which lie at the very core of the question in hand. The explanation of this complex phenomena of magne-crystallic action was rendered impossible to him through his rejection of the doctrine of diamagnetic polarity. Applying this principle to magnetic and diamagnetic crystals the force proper to each is always found acting in 'couples' in the magnetic field, and from the action of such couples the observed phenomena flow as simple mechanical consequences.

Bancalari had established the magnetism of flame. It is an interesting experiment to place a lighted candle between two pointed poles to split the flame in two by the of the magnet. According to the position of the flame it can be depressed, elevated, or blown aside, by the magnetic force. Faraday repeated Bancalari's experiments, and, passing from flames to gases generally, established their magnetic and diamagnetic powers. He made numerous experiments with oxygen and nitrogen, which, as constituents of the earth's atmosphere, had an importance of their own. Oxygen he found to be strongly magnetic, nitrogen at first feebly diamagnetic but afterwards neutral. As a boy he loved to play with soap-bubbles, and he now applied them to a more serious purpose. The deportment of oxygen in air 'was very impressive, the bubble being pulled inward, or towards the axial line, sharply and suddenly, as if the oxygen were highly magnetic.' A strong vein of metaphysics runs through the speculations of Faraday, but his experiments are always handled with regal power. He thought it important to fix the magnetic zero, to discover if possible a substance neutral to the magnet when excited to its uttermost. A bubble of nitrogen suspended in air was repelled, and a hasty observer might infer that nitrogen was diamagnetic, but Faraday saw that the apparent repulsion might be really due to the attraction of the surrounding atmospheric oxygen. After a series of experiments of the rarest beauty and precision, he came to the conclusion that nitrogen was 'like space itself' — neither magnetic nor diamagnetic.

He next compared the magnetic intensity of oxygen with that of a solution of sulphate of iron, and found that, bulk for bulk, oxygen is equally magnetic with such a solution 'containing seventeen times the weight of the oxygen in crystalised protosulphate of iron, or 3.4 times its weight of metallic iron in that state of combination.' The attraction of a bubble of oxygen at the distance of an inch from the magnetic axis he found to be about equal to the gravitating force of the same bubble. His thoughts now widen so as to embrace the earth's atmosphere and the possible action of its oxygen on the magnetic needle. Two elaborate memoirs on atmospheric magnetism were sent to the Royal Society on 9 Oct. and 19 Nov. 1850. The effect of heat and cold upon the magnetism of the air and the resultant action on the magnetic needle are discussed. Faraday here makes a masterly use of the convergence and divergence of the lines of terrestrial magnetic force. Those lines are his guiding light through this most difficult domain. He applied his results to the explanation of the annual and diurnal variation, and also considered irregular