236 HISTORY OF COLD AI^D THE ABSOLUTE ZEEO. 



earth and sun has its greatest inclination to the solar equator, so that 

 the earth is more directly exposed to the region of maximum of sun 

 spots, while the twenty-six da3"s period corresponds closely with the 

 period of rotation of that part of the solar surface where facula^ are 

 most abundant. J. J. Thomson has pointed out, as a consequence of 

 the Richardson observations, that negative ions will be constantly 

 streaming from the sun merely regarded as a hot body, but this is not 

 inconsistent with the supposition that there will be an excess, of this 

 emission in eruptions, and from the regions of faculas. Arrhenius's 

 theor}" accounts also, in a way which seems the most satisfactory 

 hitherto enunciated, for the appearances presented by comets. The 

 solid parts of these objects absorb the sun's rays, and as they approach 

 the sun become heated on the side turned toward him until the volatile 

 substances frozen in or upon them are evaporated and diffused in the 

 gaseous state in surrounding space, where they get cooled to the 

 temperature of liquefaction and aggregated in drops a])out the nega- 

 tive ions. The Jarger of these drops gravitate toward the sun and 

 form clouds of the coma al)out the head, while the smaller are driven 

 by the incidence of the sun's light upon them awa}^ from the sun and 

 form the tail. The curvature of the tail depends, as Bredichin has 

 shown, on the rate at which the particles are driven, which in turn 

 depends on the size and specific gravity of the particles, and these will 

 var}^ with the density of the vapor from which they are formed and 

 the frequency of the negative ions which collect them. In any case 

 Arrhenius's theory is a most suggestive one, not only with reference 

 to auroras and comets, and the solar corona and chromosphere, but 

 also as to the constitution of the photosphere itself. 



VARIOUS LOW TEMPERATURE RESEARCHES. 



We may now summarize some of the results which have already 

 been attained by low-temperature studies. In the lirst place, the 

 great majority of chemical interactions are entirelj^ suspended, l)ut an 

 element of such exceptional powers of combination as fluorine is still 

 active at the temperature of liquid air. Whether solid fluorine and 

 liquid hydrogen would interact no one can at present say."' Bodies 

 naturally become denser, but even 51 highly expansive substance like 

 ice does not appear to reach the density of water at the lowest tem- 

 perature. This is confirmatory of the view that the particles of mat- 

 ter under such conditions are not packed in the closest possil)le way. 

 The force of cohesion is greatl}^ increased at low temperatures, as is 

 shown b}^ the additional stress required to rupture metallic wires. 

 This fact is of interest in connection with two conflicting theories of 



«Note by author, April 16, 1903. In a recent communication to the French 

 Academy by Professor Moissan and myself it is shown that violent combinations 

 take plaet', so that chemical action can in certain cases take place near the zero of 

 temperature. [See Comptes Kendus, March 30, 1903.] 



