[Footnote 8:  Professor J.H.  Poynting, in his lecture to the British Association at Cambridge in 1904, says:  “The surface of the earth receives, we know, an amount of heat from the inside almost infinitesimal compared with that which it receives from the sun, and on the sun, therefore, we depend for our temperature.”]

In order to understand the immense significance of this conclusion we must know what is meant by the whole heat or warmth; as unless we know this we cannot define what half or any other proportion of sun-heat really means.  Now I feel pretty sure that nine out of ten of the average educated public would answer the following question incorrectly:  The mean temperature of the southern half of England is about 48 deg.  F. Supposing the earth received only half the sun-heat it now receives, what would then be the probable mean temperature of the South of England?  The majority would, I think, answer at once—­About 24 deg.  F. Nearly as many would perhaps say—­48 deg.  F. is 16 deg. above the freezing point; therefore half the heat received would bring us down to 8 deg. above the freezing point, or 40 deg.  F. Very few, I think, would realise that our share of half the amount of sun-heat received by the earth would probably result in reducing our mean temperature to about 100 deg.  F. below the freezing point, and perhaps even lower.  This is about the very lowest temperature yet experienced on the earth’s surface.  To understand how such results are obtained a few words must be said about the absolute zero of temperature.

The Zero of Temperature.

Heat is now believed to be entirely due to ether-vibration, which produces a correspondingly rapid vibration of the molecules of matter, causing it to expand and producing all the phenomena we term ‘heat.’  We can conceive this vibration to increase indefinitely, and thus there would appear to be no necessary limit to the amount of heat possible, but we cannot conceive it to decrease indefinitely at the same uniform rate, as it must soon inevitably come to nothing.  Now it has been found by experiment that gases under uniform pressure expand 1/273 of their volume for each degree Centigrade of increased temperature, so that in passing from 0 deg.  C. to 273 deg.  C. they are doubled in volume.  They also decrease in volume at the same rate for each degree below 0 deg.  C. (the freezing point of water).  Hence if this goes on to-273 deg.  C. a gas will have no volume, or it will undergo some change of nature.  Hence this is called the zero of temperature, or the temperature to which any matter falls which receives no heat from any other matter.  It is also sometimes called the temperature of space, or of the ether in a state of rest, if that is possible.  All the gases have now been proved to become, first liquid and then (most of them) solid, at temperatures considerably above this zero.