August 7, 1879] 



NATURE 



351 



action once begun is rapidly propagated, until the whole 

 solution is solid. 



In like manner newly refined fixed oils may have their 

 cohesive force degraded by various chemical means, as by 

 being treated with an alkali, or with nitric acid, &c., in 

 vvhich condition they are commonly nuclear. Solid and 

 semi-solid fats, such as suet, dripping, lard, butter, fat of 

 meat, &c., act powerfully as nuclei. The freshly cut 

 surface acts most effectually, and, in some cases, a fat cut 

 in one direction may not act for hours, while if cut in 

 another direction it may act immediately. A glass rod 

 smeared with lard, &c., may act better than a small lump 

 of the material. 



Several observers, who deny that the oils, &c., have any 

 nuclear action, so contrived their experiments as to 

 expose the oils, &c., to the action of heat, while carefully 

 excluding the outer air. The effect of such manipulation 

 is to increase the cohesive force of the bodies in question ; 

 and as they fail to act under such conditions, it was 

 denied that they ever acted at all under other conditions, 

 except when contaminated with particles of the salt more 

 or less ultra-microscopic. 



So also in testing the nuclear action of porous and 

 dehydrating bodies, heat is employed, and this destroys 

 the veiy activity which is about to be tested. Freshly 

 ignited quicklime, for example, is inclosed in thin glass 

 bulbs, sealed, heated to redness, and dropped into the 

 supersaturated solutions. When cold, the bulb is broken, 

 and the contents set free, but in no case with any result. 



I have found that porous bodies, such as pumice, 

 plaster of Paris, &c., may be rendered inactive by being 

 treated for about ten minutes in a test tube plunged into 

 boiling water. But when such bodies are thrown into the 

 solutions without any separation of salt, it is not that 

 exposure to heat has rendered them less active as nuclei ; 

 they have become in fact more active than before, for 

 they now absorb the solution as a whole instead of its 

 water only ; whereas, if after being heated, these porous 

 bodies are exposed to the outer air during ten minutes, 

 they take in a little moisture which tames down their 

 absorptive power, so that when placed in the solution 

 they act more slowly, absorbing water and thereby pro- 

 ducing a separation of salt and the solidification of the 

 solution. Absolute alcohol acts in a similar manner. 

 Crumb of bread is a very good porous nucleus. 



As to the condition of sodic sulphate in solution there 

 are many reasons for supposing that it is the anhydrous 

 salt ; that is, the normal salt gives up the whole of its 

 water of crystallisation to the solution. When by a 

 reduction of temperature a portion of the salt is separated, 

 it combines with seven atoms of water, and this is not 

 nuclear. So also when nuclei fail to effect the solidifica- 

 tion of the solution, they cannot be said to be inactive, 

 since they act within certain limits of temperature by 

 throwing down the modified salt. But when nuclei are 

 active in determining solidification they form a kind of 

 point d'appui, which enables the disengaged molecule 

 to combine with ten atoms of water, and this is nuclear 

 to the rest of the solution and determines its solidification. 



The state of supersaturation is dependent to a con- 

 siderable extent on the adhesion of the solution to the 

 walls of the vessel. If a portion of this be detached by 

 rubbing or scratching the side below the surface with a 

 strong clean wire the whole system breaks down and the 

 solution immediately becomes solid. If the vessel, such 

 as a test tube, be lined with resin, amber, or some sub- 

 stance to which the solution has a weaker adhesion, it 

 generally becomes soHd in cooling. Or if a highly super- 

 saturated solution be poured boiling hot into a shallow 

 vessel, in which the surface of adhesion and the free 

 surface are nearly equal, the solution relieves itself by 

 throwing down a considerable quantity of the modified 

 salt, at temperatures above that at which it is usually 

 formed in nairovver and deeper vessels. 



I have thus very briefly stated the results of my re- 

 searches on this difficult subject, and submit them to the 

 candid judgment of fellow-labourers in the same field. 



C. TOJILINSON 



OUR ASTRONOMICAL COLUMN 

 Venus in the Pleiades.— A thousand years ago and 

 later the geocentric track of the planet Venus was occa- 

 sionally such as to cause it to traverse the Pleiades, a 

 phenomenon which, in these telescopic days, would be 

 one of no small interest to the observer ; prior to the 

 invention of the telescope its effect would be merely to 

 obliterate in a great measure, for the time being, the stars 

 forming this group, particularly when the planet happened 

 to be upon them near an epoch of greatest brilliancy. 

 Amongst the observations collected from the Chinese 

 annals and forwarded to Europe in the middle of the last 

 century by the French Jesuit, Gaubil, we find one made 

 under the dynasty Tang, in the fifth year of the period 

 Hwuy-Chang, on the day yin- Woo, of the second moon 

 — corresponding in the Julian Calender to a.d. 845, 

 March i5 — when "Venus eclipsed the Pleiades;" the 

 observation appears to have been made at Si-gan-fou, 

 where the Tang dynasty had their court, and where the 

 earliest occultation of a star or planet by the moon, that 

 of Mars B.C. 69, February 14, was also observed. If we 

 found an examination of this " eclipse " of the Pleiades 

 by Venus, upon Leverrier' s data, using them with a suffi- 

 cient degree of approximation for the purpose in view, it 

 appears that at dusk at Si-gan-fou, on March 16, 845, 

 Venus was situate near the star Electra, not far from the 

 parallel of the principal star of the group Alcyone, but 

 three-quarters of a degree to the west of it, and that about 

 the same time on the evening of March 17, after having 

 passed close to Maia, the planet would be in the same 

 right ascension as Alcyone, about twenty-four minutes to 

 the north. Although the so-called eclipse of the Pleiades 

 might commence therefore on March 16 as the Chinese 

 record, Venus would be more centrally upon the group 

 on March 17, according to our modern tables. Her 

 apparent diameter was then thirty seconds, and she 

 would shine with more than average brilliancy. Anotlier 

 eclipse of the Pleiades is mentioned under the later Sung 

 dynasty, on March 10, A.D. 1002. 



Varro's Story of the Anomalous Track and 

 Figure of Venus. — In a rhume of the recent progress 

 of astronomy contributed to an American work by Prof. 

 Holden, of Washington, we note a reference to a com- 

 munication made by M. Boutigny to the Academy of 

 Sciences at Paris in December, 1877, calling attention to 

 a passage in Varro, which describes the planet Venus, as 

 having about the year B.C. 1831 (not B.C. 31, as mis- 

 printed in Prof. Holden's Report) " changed its diameter, 

 colour, figure, and course." M. Boutigny had probably 

 overlooked the circumstance that this story of Varro's had 

 been brought into notice long before, in a French work 

 of astronomical authority, the " Cometographie " of 

 Pingrd, who, believing that the fable was originated by 

 some celestial phenomenon, considered it was most 

 probably due to the appearance of a bright comet. 

 Pingrd thus gives the fragment, preserved by St. 

 Augustin : — " There was seen, says Varro, a surprising 

 prodigy in the heavens, with regard to the brilliant star 

 Venus, which Plautus and Homer call, each in his own 

 language — the evening star. Castor affirms that this fine 

 star changed its colour, size, figure, and track, which had 

 never occurred before, and which has not occurred since. 

 Adrastus, of Cyzicus, and Dion the Neapolitan, refer this 

 great prodigy to the reign of Ogyges." Pingrd's ex- 

 planation will be found in "Cometographie," t. i. p. 247 ; 

 the epoch he assigns for the phenomenon is "vers 1770." 

 The story has no astronomical importance, and is only 

 noticed here from its revival so recently, as mentioned 

 above. 



