April 27, 1905] 



NA TURE 



609 



P orbit. The deviation amounts to but 3°, and its plane of 

 f rotation lias tlierefore shifted through 177°. 

 I The explanation of the retrograde rotation of Phcebe is 

 I now also clear. Phoebe, the first-born of Saturn's 

 numerous retinue, came into being while the planet itself 

 still retained its original plane of rotation, that is, while 

 it was still revolving in a retrograde direction. Before 

 lapetus, Saturn's second satellite, reckoning from without 

 inwards, was created, the mighty tides acting upon the 

 planet in its then diffuse condition had shifted its plane of 

 rotation more than 90°. Two forces then acted on the 

 plane of the orbit of the new satellite, one from the sun 

 tending to bring the orbit into the plane of the orbit of 

 Saturn, the other from Saturn tending to bring the orbit 

 of the satellite into the plane of the equator of its primary. 

 At first both forces tended to produce the same result, 

 namely, to diminish the angle of inclination of the plane 

 of the orbit of the satellite. They are now pulling in 

 opposite directions, as is the case with our own moon, the 

 inclination of the orbit of lapetus, iq°, being less than 

 that of the equatorial plane of its primary. 



The inner satellites of Saturn are more powerfully 

 affected by the equatorial expansion of the planet than by 

 the action of the sun, the planes of their orbits, 27°, coin- 

 ciding nearly with the plane of the planet's equator. 



William H. Pickerin'G. 

 Harvard Observatory, Cambridge, Mass., U.S. .A. 



riave Chemical Compounds a Definite Critical 

 Temperature and Pressure of Decomposition ? 



So far nobody seems to have considered the question 

 whether to every chemical compound there exists a definite 

 critical temperature and pressure of decomposition. Yet 

 I think the following considerations show that such con- 

 stants probably do exist. Suppose we place a given com- 

 pound (say CaCOj) in a closed cylinder and subject it to 

 a continually increasing temperature, keeping the pressure 

 constant by means of a weighted piston. Then at a 

 certain definite temperature range the compound will begin 

 to decompose. Suppose, now, we increase the pressure 

 sufficiently ; then the decomposition ceases, and the sub- 

 stance can now bear a higher temperature than before 

 without decomposition. 



Proceeding in this way, it is, I think, obvious from the 

 finite nature of the mass of the atoms, and from the 

 limited intensity of the forces holding them together in 

 the molecule, that ultimately at some definite finite 

 temperature the external forces tending to drive the atoms 

 apart will become equal to the maximum internal forces 

 that the atoms can exert on each other in the molecule. 

 It therefore follows that above a certain definite tempera- 

 ture, depending upon the nature of the molecule, no 

 pressure, however great, can prevent the substance from 

 completely decomposing. This temperature and pressure, 

 above which a compound is incapable of existing, we will 

 call the critical temperature and pressure of decomposition 

 of the compound. The critical temperature and pressure of 

 decomposition would therefore be completely analogous to 

 the critical temperature of liquefaction of a compound — 

 only in the latter case we are dealing with the temperature 

 whereat a certain molecular condition of existence dis- 

 appears, and in the former case with the temperature 

 whereat a certain atomic condition of existence disappears. 



Since atoms are a very much more finely divided form 

 of matter than molecules, it is clear that the critical 

 temperature of decomposition of a compound must be a 

 very much sharper and clear-cut constant than its critical 

 temperature of liquefaction. The critical temperature and 

 pressure of even very unstable compounds is usually very 

 high, provided there exist but a few atoms in the molecule. 

 For example, AuC!,, ozone, and the oxides of nitrogen, 

 although very unstable at ordinary temperatures, seem 

 capable of existing at very high temperatures. In general, 

 the greater the number of atoms contained in the molecule 

 the lower the critical temperature of decomposition, as is 

 evident from the general observation that the more com- 

 plex a compound is the easier it is to decompose. Manv 

 of the very complex carbon compounds — for example, the 



NO. 1852, VOI. 71] 



proteids — have, on account of their complexity, critical 

 temperatures of decomposition which lie very close to the 

 normal temperature of the earth's surface. 



If, now, by some means we proceed to add on atoms 

 to such a molecule so as to make it more and more com- 

 plex, we would steadily lower its critical temperature of 

 decomposition, and by adding on a suitable kind and 

 number of atoms we could reduce the critical temperature 

 and pressure of the compound until they coincided with 

 the normal temperatures and pressures which hold upon 

 the earth's surface. Such a compound would be possessed 

 of an extraordinary sensitiveness to external influences on 

 account of the sharpness of the constants called above the 

 critical temperature and pressure of the compound. The 

 slightest increase of temperature or decrease of pressure 

 would serve to throw it into a condition of rapid chemical 

 decomposition, whereas a slight increase of pressure and 

 decrease of temperature would cause it to cease to de- 

 compose. Even did we maintain the external temperature 

 and pressure exactly at the critical temperature and 

 pressure of the compound, nevertheless the external im- 

 pulses which are continuously pervading all space in the 

 neighbourhood of tne solar system, beating intermittently 

 upon the sensitive substance, would be sufficient to throw 

 it into a series of rapidly alternating states of decomposi- 

 tion and repose. 



I suggest that the temperature range of animal life is 

 probably nothing more or less than the range of the critical 

 temperatures of decomposition of a series of certain very 

 complex carbon compounds which are grouped together 

 under the name "protoplasm," the external pressure of 

 the atmosphere coinciding roughly with their critical 

 pressures of decomposition. In fact, I suggest that just 

 as a tuning-fork is set into motion by vibrations of a 

 certain definite frequency and by no others, so living 

 matter is so constructed as to respond continuously to the 

 incessant minute fluctuations in the external conditions 

 which hold upon the earth, the state of response being 

 what is known as life. The temperature of animal life 

 keeps remarkably constant, as it should do on our sup- 

 position, a temperature too high exceeding the critical 

 temperature of decomposition of living matter and so 

 destroying its structure, while a temperature too low causes 

 it to cease to decompose, and the living matter becomes 

 inactive. Geoffrey M.^rtin. 



University of Kiel, .\pril 4. 



[The writer of the above will see his " suggestion " dis- 

 cussed in Lockyer's "Inorganic Evolution," book iii. — 

 En. Nature. 1 



Experiment on Pressure due to Waves. 



I HAVE seen both in the Physikalische Zeiischrip 

 (January) and in the Physical Heview (February) an 

 account of an experiment by Prof. R. W. Wood to demon- 

 strate the pressure due to waves, and which he suggests 

 as a lecture demonstration of the effect observed by 

 Lebedeff and by Nichols and Hull. The same experiment 

 is quoted by Prof. Poynting in his address on this subject 

 to the Physical Society of London (Phil. Mag., April). I 

 venture to suggest that the experiment, which consists in 

 setting a small windmill in motion by means of Leyden 

 jar discharges maintained by a transformer, will bear a 

 different explanation. It was shown long ago (1703) by 

 Kinnersley, of Philadelphia, in his " Electrical Thermo- 

 meter," that a jar discharge produces in air a violent ex- 

 plosive effect, which we should now explain by the re- 

 pulsion between constituents of the current in opposite 

 phase to one another. The repulsive force may be very 

 great. I think it is this explosive effect that Prof. Wood 

 shows in the experiment, and not the pressure due to reflec- 

 tion of a continuous train of waves. I do not think that 

 the suggestion is new, but it appears to me that the same 

 cause may account for the disruption which occurs when 

 lightning strikes a building, an instance of which is re- 

 corded in Nature of .April 13 (p. 565) in the displacement 

 of some of the blocks of the small pyramid. 



Sidney Skinner. 



.South-Western Polytechnic, Chelsea, .April 15. 



