Page:EB1911 - Volume 06.djvu/873

Rh orifice. On account of the expansion resulting from this sudden decrease of pressure, the temperature of the gas, and consequently of the two spirals, falls sensibly. If this process is repeated with another current of gas, this current, having been cooled in the inner spiral, will be cooled still further, and the temperature of the two spirals will become still lower. If the pressures p1 and p2 remain constant the cooling will increase with the lowering of the temperature. In Linde’s apparatus this cycle is repeated over and over again, and after some time (about two or three hours) it becomes possible to draw off liquid air.

The cooling which is the consequence of such a decrease of pressure was experimentally determined in 1854 by Lord Kelvin (then Professor W. Thomson) and Joule, who represent the result of their experiments in the formula

In their experiments p2 was always 1 atmosphere, and the amount of p1 was not large. It would, therefore, be certainly wrong, even though for a small difference in pressure the empiric formula might be approximately correct, without closer investigation to make use of it for the differences of pressure used in Linde’s apparatus, where p1 = 200 and p2 = 18 atmospheres. For the existence of a most favourable value of p1 is in contradiction with the formula, since it would follow from it that T1 – T2 would always increase with the increase of p1. Nor would it be right to regard as the cause for the existence of this most favourable value of p1 the fact that the heat produced in the compression of the expanded gas, and therefore p1/p2, must be kept as small as possible, for the simple reason that the heat is produced in quite another part of the apparatus, and might be neutralized in different ways.

Closer examination of the process shows that if p2 is given, a most favourable value of p1 must exist for the cooling itself. If p1 is taken still higher, the cooling decreases again; and we might take a value for p1 for which the cooling would be zero, or even negative.

If we call the energy per unit of weight and the specific volume v, the following equation holds:—

1 + p1v1 – p2v2 = 2,

or

1 + p1v1 = 2 + p2v2.

According to the symbols chosen by Gibbs, 1 = 2.

As 1 is determined by T1 and p1, and 2 by T2 and p2, we obtain, if we take T1 and p2 as being constant,

If T2 is to have a minimum value, we have

From this follows

As (1/v1)T is positive, we shall have to take for the maximum cooling such a pressure that the product pv decreases with v, viz. a pressure larger than that at which pv has the minimum value. By means of the equation of state mentioned already, we find for the value of the specific volume that gives the greatest cooling the formula

and for the value of the pressure

If we take the value 2Tc for T1, as we may approximately for air when we begin to work with the apparatus, we find for p1 about 8pc, or more than 300 atmospheres. If we take T1 = Tc, as we may at the end of the process, we find p1 = 2.5pc, or 100 atmospheres. The constant pressure which has been found the most favourable in Linde’s apparatus is a mean of the two calculated pressures. In a theoretically perfect apparatus we ought, therefore, to be able to regulate p1 according to the temperature in the inner spiral.

The critical temperatures and pressures of the permanent gases are given in the following table, the former being expressed on the absolute scale and the latter in atmospheres:—

The values of Tc and pc for hydrogen are those of Dewar. They are in approximate accordance with those given by K. Olszewski. Liquid hydrogen was first collected by J. Dewar in 1898. Apparatus for obtaining moderate and small quantities have been described by M. W. Travers and K. Olszewski. H. Kamerlingh Onnes at Leiden has brought about a circulation yielding more than 3 litres per hour, and has made use of it to keep baths of 1.5 litre capacity at all temperatures between 20.2° and 13.7° absolute, the temperatures remaining constant within 0.01°. (See also .)

CONDENSER, the name given to many forms of apparatus which have for their object the concentration of matter, or bringing it into a smaller volume, or the intensification of energy. In chemistry the word is applied to an apparatus which cools down, or condenses, a vapour to a liquid; reference should be made to the article for the various types in use, and also to (Gas Manufacture) and ; the device for the condensation of the exhaust steam of a steam-engine is treated in the article. In woollen manufactures, “condensation” of the wool is an important operation and is accomplished by means of a “condenser.” The term is also given—generally as a qualification, e.g. condensing-syringe, condensing-pump,—to apparatus by which air or a vapour may be compressed. In optics a “condenser” is a lens, or system of lenses, which serves to concentrate or bring the luminous rays to a focus; it is specially an adjunct to the optical lantern and microscope. In electrostatics a condenser is a device for concentrating an electrostatic charge (see ;

 CONDER, CHARLES (1868–1909), English artist, son of a civil engineer, was born in London, and spent his early years in India. After an English education he went into the government service in Australia, but in 1890 determined to devote himself to art, and studied for several years in Paris, where in 1893 he became an associate of the Société Nationale des Beaux-Arts. About 1895 his reputation as an original painter, particularly of Watteau-like designs for fans, spread among a limited circle of artists in London, mainly connected first with the New English Art Club, and later the International Society; and his unique and charming decorative style, in dainty pastoral scenes, gradually gave him a peculiar vogue among connoisseurs. Examples of his work were bought for the Luxembourg and other art galleries. Conder suffered much in later years from ill-health, and died on the 9th of February 1909.

 CONDILLAC, ÉTIENNE BONNOT DE (1715–1780), French philosopher, was born at Grenoble of a legal family on the 30th of September 1715, and, like his elder brother, the well-known political writer, abbé de Mably, took holy orders and became abbé de Mureau. In both cases the profession was hardly more than nominal, and Condillac’s whole life, with the exception of an interval as tutor at the court of Parma, was devoted to speculation. His works are Essai sur l’origine des connaissances humaines (1746), Traité des systèmes (1749), Traité des sensations (1754), Traité des animaux (1755), a comprehensive Cours d’études (1767–1773) in 13 vols., written for the young Duke Ferdinand of Parma, a grandson of Louis XV., Le Commerce et le gouvernement, considérés relativement l’un à l’autre (1776), and two posthumous works, Logique (1781) and the unfinished Langue des calculs (1798). In his earlier days in Paris he came much into contact with the circle of Diderot. A friendship with Rousseau, which lasted in some measure to the end, may have been due in the first instance to the fact that Rousseau had been domestic tutor in the family of Condillac’s uncle, M. de Mably,