110 



• KNOWLEDGE • 



[Dko. 9, 1881 



lulditioiiat temperature which is reciuirtxl to overcome an 

 tulditioual atmosphere of j)ressiire, aiul if this goes on as 

 tlie pressure ami t«>niperatures advance, we may ultiniatcly 

 reach a curious condition — a temperature at whicii addi- 

 tional pressure will demand no additional temperature to 

 maintain the gaseous state ; or, in other words, a t<>mpera- 

 Lure may be reached at which no amount of jiressurc can 

 condense steam into water, or where the gaseous and liquid 

 states merge or become inditl'erent. 



But we must not push tliis mere numerical reasoning 

 too far, seeing that it is quite possible to be continually 

 ;ipproaching a given point, without ever reaching it, as 

 when we go on continually halving the remaining distance. 

 The figures in the al)0ve do not appear to follow according 

 to such a law — nor, indeed, any other regularity. This pro- 

 bably arises from experimental error, as there are dis- 

 crepancies in the results of diflerent investigators. They 

 all agree, however, in the broad fact of the gradation above 

 stated. Dulong and Arago, who directed the experiments 

 of the French Government Commission for investigating 

 this subject, state the pressure at 20 atmo.sphercs to be 1181 

 at 21=4-i20, at •J.3 = 427-3, at 2.'5 = 431-4, and at 24 atmo- 

 spheres, their highest c.vperi mental, limit 435-5, thus re- 

 ducing the rise of temperature between the 23rd and 24th 

 atmosphere to 4'1. 



If we could go on heating water in a transparent vessel 

 until this ditterence became a vanishing quantity, we should 

 probably recognise a visible physical change coincident with 

 this cessation of condensibUity by pressure ; but this is not 

 possible, as glass would become red-hot and softened, and 

 thus incapable of bearing the great pressure demanded. 

 Besides this, glass is soluble in water at these high 

 temperatures. 



If, however, we can find some liquid with a lower boiling- 

 point, we may go on piling atmosphere upon atmosphere of 

 elastic expansive pressure, as the temperature is raised, 

 without reaching an unmanageable degree of heat. Liquid 

 carbonic acid, which, under a single atmosphere of pressure, 

 boils at 112° below the zero of our thei'mometer, may thus 

 be raised to a temperature having the same relation to its 

 boiling-point that a red heat has to that of water, and may 

 be still confined within a glass vessel, provided the walls of 

 the vessel arc sufficiently thick to bear the strain of the 

 el.istic outstriving pressure. In .spite of its brittleness, 

 glass is capable of bearing an enormous strain steadily 

 applied, as may be proved by trying to break even a mere 

 thread of glass liy direct pull. 



Dr. Andrews thus treated carbonic acid, and the ex- 

 periment, as I have witnessed its r(>petition, is very curious. 

 A liquid occupies the lower part of a very strong glass 

 tube, which appears empty above. But this apparent void 

 is occupied Ijy invisible carbonic acid gas, evolved by the 

 previous boiling of the liquid carbonic acid below. We 

 start at a low temperature — say 40° Fahr. Then the 

 temperature is raised ; the liquid boils until it has given oil' 

 sufficient gas or vapour to exert the full expansive pressure 

 or tension due to that temperature. This pressure stops 

 the boiling, and again the surface of the liquid is becalmed. 

 This is continued until we approach nearly to 88° Fahr., 

 when the surface of the liquid loses some of its sharp 

 outline. Then 88° is reached, and the boundary between 

 liquid and gas vanishes ; liquid and gas have blended into 

 one mysterious intermediate fluid ; an indefinite fluctuating 

 something is there filling the whole of the tube — an 

 etherealised liquid or a visible gas. Hold a red-hot poker 

 between your eye and the light ; you will sec an upflowing 

 wavy movement of what appears like liquid air. The 

 appearance of the hybrid fluid in the tube resembles this, 

 but is sensibly denser, and evidently stands between the 



liquid and gaseous states of matter, as pitch or trea. '.■■ 

 .stands 1)ctween solid and liquid. 



The temperature at which this occurs has been named 

 by Dr. Andrews the "critical temperature ;" here the 

 gaseous and li(|uid states are " continuous," and it is 

 probable that all other sub.stances capable of existing iu 

 both states have their own particular critical temperatures. 



Having thus stated the facts in popular outline, I shall 

 conclude the subject in my next paper by indulging in 

 some speculations of my own on the philosophy of tliesf 

 general facts or natural laws, and on some of their pos-sibl.- 

 consequences. 



PERSPECTIVE ILLUSIONS. 



Bv H. J. S=!l.\ck, F.G.S., Ac. 



SOME persons have very little perception of perspective, 

 and whole nations, as the Chinese, whether or not 

 impressed by its effects, are not offended by drawings made 

 in defiance of its rules. If experiments are made with 

 English folk of diflerent ages and degrees of education, a 

 large proportion will be found obtuse in perceiving, and 

 inaccurate in observing, the optical aspects of buildings, 

 pieces of furniture, crockery, ic, as seen from different 

 positions. On the other hand, the more artistically-cul- 

 tivated or more naturally-endowed persons are extremely 

 sensitive to all such effects ; and, in some cases, most easily 

 deceived. Etchings in simple lines, without shading, such 

 as Flaxman's illustrations of Dante, show how easily the 

 eye is led, by slight tliickenings, curves, and angular 

 approximations, to conceptions of distance or super- 

 position, and it is probable that the illusions thus pro- 

 duced are strongest in the sharpest observers. Persons are 

 sometimes met with who see nothing of the kind, and to 

 whom all drawings look flat. Illusion figures are most 

 deceptive when they give no hint of the real facts. Thus, 

 your Fig. 8, p. 70, deceives an observer who finds no illu- 

 sion in Fig. 9 on the opposite page. I see the illusions in 

 both, but strongest in Fig. 8. Fig. 9 is more like a mosaic 

 pavement, in regard to which the knowledge that the 

 surface is flat makes it seem so, in spite of the Unes. In 

 looking at pictures and engravings, the mind willingly 

 accepts their perspective indications. In Fig. 8 the illu- 

 sion is produced by the series of diminishing arcs from the 

 edge of the outer circle inwards. These correspond suffi- 

 ciently with the representation in perspective of a series 

 of curved objects of the same size, but looking smaller and 

 smaller as the distance increases. This makes the centre 

 of the line AB appear to bend inwards. 



Compound vibration curves described in fine lines on 

 paper are very deceptive. Mr. Washington Teasdale, who 

 has great skill in these matters, has supplied me with many 

 beautiful illustrations of this fact. He has also enabled 

 me to make a variety of experiments with similar patterns, 

 minutely ruled on glass as microscopic objects. The optical 

 illusions are strongest when the lines correspond with those 

 used in perspecti^•e di-awing ; and high magnification 

 (say 1,000 x ) does not dissipate the impression, if enough 

 of the pattern remains visible in the diminished field. 

 When bands of lines of the same thickness cross each 

 other, it is extremely difficult to illustrate them so as to 

 make them look on one plane. Of two such series, either 

 may be made to look uppermost. With large angle of 

 operation and high powers, I believe it is quite impossible 

 to obtain certain information of the structure of many 

 objects, unless there are good reasons, independent of 

 vision, for supposing one optical appearance more corre- 

 spondent with fact than another. 



