Oct. 26, 1 88 2 J 



NA TURE 



639 



owing to the decomposition taking place in such a solution, still 

 the green colour from the modified chlorophyll will long remain. 

 A single drop of hydrochloric acid added to the green extract, 

 although it at once changes the bright green to a darker and 

 browner preen, enables the solution to resist this action of light 

 to a much greater extent than it could have done if no acid had 

 been added. 



In the one-banded modification of chlorophyll we appear to 

 have a body on which light has no action; solutions of this 

 hody have been, for the last three months, exposed continuously 

 to all the light and sunshine w*e could get, and they are un- 

 changed in colour and constitution ; another proof of the really 

 wonderful stab lity of this substance. Again, as a confirmation 

 of the properties and formation of this form of chlorophyll, a 

 single drop of sulphate of copper added to an ordinary 

 chlorophyll extract renders the green colour of the solution 

 permanent. 



The very striking change of tint which occurs when a strong 

 chlorophyll solution is very considerably diluted, whereupon it 

 changes from a dark to a light yellowish green, forcibly suggests 

 to us the probability that the difference in shade of old leaves as 

 compared with young ones, is due to the same cause, namely, 

 the greater or smaller amount of chlorophyll in a given area. 



ON A METHOD OF INVESTIGATING EX- 

 PERIMENTALLY THE ABSORPTION OF 

 RADIANT HEAT BY GASES « 



'"THERE are grave objections, which have been only partially 

 •^ overcome, to almost all the processes hitherto employed for 

 testing the diathermancy of vapours. These arise chiefly from 

 conden-ation on some part of the apparatus. Thus when rock- 

 salt is used, an absorbent surface-layer may be formed ; and, 

 when the pile is used w ithout a plate of salt, the effect of radiant 

 heat may be to cool it (the pile) by the evaporation of such a 

 surface film. The use of intermittent radiation is liable to the 

 same objection. 



Some time ago it occurred to me that this part of the difficulty 

 might be got rid of by dispensing with the pile, and measuring 

 the amount of absorption by its continued effects on the volume 

 and pressure of the gas or vapour itself. 



Only preliminary trials have, as yet, been made. They were 

 carried out for me by Prof. Mac-Gregor and Mr. Lindsay. 



Their object was first to find whether the method would work 

 well, second (when this was satisfactorily proved) to find the best 

 form and dimensions for the apparatus. 



The rough apparatus is merely a double cylinder, placed ver- 

 ticals. Cold water circulates in the jacket, and steam can be 

 blown into the double top. The changes in the pressure of the 

 gas are shown by a manometer (J tube at the bottom, which 

 contains a liquid which will not absorb the contents. This 

 apparatus was 4 feet long, with 2 inches internal radius. The 

 resubs of a number of experiments show that i should be shorter 

 and much wider. The former idea I was not quite prepared for, 

 the latter is obvious. 



'1 he effects on the manometer are due to five chief causes : — 



1. Heating of the upper layer of gas by contact with lid. 



2. Cooling ,, ,, ,, ,, sides. 



3. Hea'ii g of more or less of the column by absorption. 



1 I elter from Prjf. Tail, read by Sir W. Thomson at the Southampton 

 meeting of the British Association. 



4. Cooling of do. by radiation. 



5. ,, ,, contact. 



(1) and (a) only are present in a perfectly diathermancus gas, 

 and in a perfectly adiathermanous gas or vapour. 



All five are present in a partially diathermancus gas or vapour. 



The preliminary experiments show that the manometer effect 

 is only very slightly less for dry olefiant gas than for dry air, 

 while moist air shows a markedly smaller effect than either of 

 the others. 



This is conclusive as to the absorption of low radiant heat by 

 aqueous vapour, but it shows also that the absorption is so small 

 as to take place throughout the whole column. 



Even with the present rude apparatus I hope soon to get a 

 very accurate determination of the absrrbirg power of aqueous 

 vapour, by finding in what proportions olefiant gas must be 

 mixed with air to form an absorbing medium equivalent to 

 saturated air at different temperatures. 



I have to acknowledge valuable hints from Prof. Stokes, who, 

 before I told bim the results I had obtained (ihus know ing merely 

 the nature of the experiments) made something much higher than 

 a guess) though somewhat short of a prediction, cf the truth. 



In these preliminary trials no precaution was taken to exclude 

 dust. The results, therefore, ate still liable to a certain amount 

 of doubt, as Mr. Aitken's beautiful exr eriments have shown. 



The point of the method is that there can be no question of 

 surface layers. 



[Since the above was written, Messrs. Mac-Gregor and Lindsay 

 have made an extended series of experiments with dry and moist 

 air, and with mixtures of dry air and olefiant gas in different 

 proportions. The cylinder em] loved was 9 inches in radius. 

 The results will soon be communicated to the Royal Society cf 

 Edinburgh.— P. G. T.] 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



Oxford. — In addition to the courses in Natural Science de- 

 scribed in last week's NATURE, the following will be given 

 during the present Michaelmas term: — Prof. Pritchard will 

 give a course of six lectures en the Theory of the Transit 

 Instrument, Equatorial, and Sextant, to be followed by six 

 lectures on the- Lunar Theory. There will be eight lectures 

 on Instrumental Practice, and eight "Evenings with the Tele- 

 scope," the latter being of a popular and unteebnical character. 



Prof. Law-son has announced the following courses of lectures 

 for the ensuing year at the Botanical Gardens : — 



Course I. Vegetable Histology ; Michaelmas Term, 1882. 



Course II. Special Morphology ; Lent Term, 1883, and 

 Trinity Term, 1883 (continued). 



Course III. Descriptive Botany; every Saturday in Lent and 

 Trinity Terms, 1882. 



Prof. Prestwich gives a course on Theoretical Geology at the 

 University Museum, and Prof. Westwocd on Certain Groups of 

 Anthropcda. 



The Regius Professor of Medicine gives notice that an 

 examination for certificates in Preventive Medicine and Public 

 Health will be held this term, and secondly that Bachelors of 

 Medicine may proceed to the degree of Doctor in any term, on 

 due notice being given. 



Natural Science Scholarships are offered this term at Ealliol 

 and at Christ Church. 



The notice issued by Balliol College states there will be an 

 election to a sholarship on the foundation of Miss Hannah 

 Brackenbury, "for the encouragement of the study of Natural 

 Science," worth 80/. a year (55/. and tuition free), tenable 

 during residence for four years : open to all such candidates as 

 shall-not have exceeded eight terms from Matriculation. This 

 examination will begin on Thursday, November 16, at ten 

 o'clock. Papers will be set in the following subjects: — (1) 

 Mechanical Philosophy and Physics ; (2) Chemistry; (3) Biology. 

 But candidates will not be expected to offer themselves in more 

 than two of these. There will be a practical examination in one 

 or more of the above subjects, if the examiners think it expedient. 

 There will also be an optional paper in Mathematics ; and the 

 literary qualifications of the candidates will be tested by an 

 English essay, or by a paper of general questions. 



At Christ Church at least one scholar will be elected in Natu- 

 ral Science. Papers will be set in Biology, Chemistry, and 

 Physics, but no candidate will be allowed to offer more than two 

 of these subjects. An optional paper will be set in Elementary 



