2l8 



NATURE 



\yuly 15, 1875 



of the vessel is about a litre or 1000 cubic centimetres. 

 But since we know that the eichaustion has reduced the 

 density to -4Trui7rT7r of its original, the volume occupied by 

 the residual gas at ordinary pressures would be that of a 

 little bubble ^\-^ of an inch in diameter. 



Sir Wm. Thomson, T. Clerk-Maxwell, and Clausius have 

 shown that in a gas, at ordinary pressure, the mean or average 

 path between two collisions is about to W of a millimetre. 

 When tke pressure is reduced to ttjo oo oo the mean will be 

 400 millimetres, or about a foot and a half. What takes 

 place is this. The particles of the gas are flying about in 

 all directions, with a velocity which depends on the tem- 

 perature. When they impinge on the heated disc their 

 velocity is increased, they go off with a greater velocity 

 than those which go off from the colder side, and hence 

 there is a recoil of the disc. When the gas is at all dense 

 the particles get a very short way before they are met by 

 another and sent back, and so the velocity gets a common 

 velocity before any visible action takes place. When the 

 gas is rare the particles may get a long way off before they 

 meet others, and so the action becomes perceptible. 



In case of cooling they go away with diminished ve- 

 locity and a negative recoil. 



The author of the paper went on to show that the total 

 mechanical action on a square centimetre of black surface 

 derived from the radiation of a magnesium lamp, at a dis- 

 tance of 150 mill., did not exceed a continuous pressure of 

 JLj part of a milligramme, and that the total work done 

 did not amount to the five-millionth part of the available 

 energy received by the movable surfaces. 



ADDITION TO OUR KNOWLEDGE OF THE 

 TERMITES* 



FRITZ MULLER has recently pubHshed a short but 

 interesting memoir on the larvae of Calotermes, 

 a genus of Termites, which he describes with his wonted 

 care and accuracy. We cannot, of course, here follow 

 him in detail ; but, as is so often the case in the writings 

 of this eminent naturalist, he draws our attention by his 

 descriptions to several points of unusual interest. As 

 occurs in some other insects, the youngest larvse of 

 Calotermes differ much in form from those somewhat 

 more advanced in age. The form of the younger larvne 

 may be accounted for on two hypotheses. It may be an 

 adaptation to the mode of life, or it may be the original 

 larval form of the group. In the latter case, Herr Miiller 

 considers that it would be an extremely interesting form, 

 because, in his opinion, Calotermes is one of the oldest, 

 if not the oldest, of existing insect genera ; since, accor- 

 ding to Hagen, the carboniferous Termites described by 

 Goldenburg from the cold strata belong to this group. 

 Under the latter hypothesis, therefore, the younger larvae of 

 Calotermes would have, as regards insects, an interest 

 similar to that possessed by Nauplius among Crustacea ; 

 and, according to Miiller, the latter really is the case. 

 The youngest larvae of Calotermes live with their elder 

 sisters, in the same localities, on the same food, and, in fact, 

 under precisely the same conditions. These older larvae 

 have, in a word, completely adapted themselves to their 

 dwelling-place and mode of life. Like most animals 

 which burrow in earth, wood, or stone, they are cylindrical 

 in form. Not so the youngest larvEe, which are flattened, 

 and have the thorax laterally expanded. Their structure 

 is, in Miiller's opinion, as unsuitable as possible for 

 animals inhabiting wood. This form is therefore pro- 

 bably only possessed through inheritance from far distant 

 ancestors. 



It is unnecessary to point out how great is the interest 

 attaching to these larva;, if Miiller's view be correct ; 

 nor would I venture to express any dissent from his con- 

 clusions. But, I confess, there seems to me a difficulty 



» By Fritz MOUer. 



in comprehending why the younger larvae have not 

 adapted themselves to their conditionSj'^in like manner as 

 their elders. 



May there not possibly be some circumstances which 

 have hitherto escaped observation, and which might 

 render the form of these larvae not so altogether unsuit- 

 able as Miiller supposes ? 



I will just refer to one other point in this interesting 

 paper. The author shows that the main, if not the whole 

 growth of the antenna takes place in the third segment : 

 the two basal ones and the terminal portion remaining 

 almost unaltered. My husband, many years ago (Linn. 

 Trans., 1863), showed this to be the case in the 

 Ephemera {Chloeon), and it would be interesting to know 

 whether the same thing occurs among other Neuroptera. 



High Elms Ellen Lubbock 



NOTES 

 The Loan Exhibition of Scientific Apparatus at South 

 Kensington, to which we have already referred (vol. xi. p. 301), 

 will open on the 1st of April, 1876, and remain open until the 

 end of September, after which time the objects will be returned 

 to the owners. It will, as we have already intimated, consist of 

 instruments and apparatus employed for research, and other 

 scientific purposes, and for teaching. It will also include appa- 

 ratus illustrative of the progress of science, and its application 

 to the arts, as well as such as may possess special interest on 

 account of the persons by whom, or the investigations in which, 

 it had been employed. The precise limits are detailed under 

 several sections in a syllabus which has been issued for the in- 

 formation of exhibitors. Models, drawings, or photographs will 

 also be admissible where the originals cannot be sent. The 

 apparatus may, in certain cases, be arranged in train as used for 

 typical investigations ; and arrangements will be made, as far as 

 it may be found practicable, for systematically explaining and 

 illustrating the use of the apparatus in the various sections. 

 Forms on which to enter descriptions of objects offered for 

 exhibition may be obtained on application to the Director of the 

 South Kensington Museum, London, S.W. These forms should 

 be filled up and returned as soon as possible, so that exhibitors 

 may receive early intimation as to the admissibility of the objects 

 they propose to send. The cost of carriage of all objects 

 selected for exhibition will be defrayed by the Science and Art 

 Department. It is hoped that institutions or individuals having 

 instruments of historic interest will be good enough to lend 

 them. The following are the various sections into which the 

 Exhibition will be divided : — Arithmetic, Geometry, Measure- 

 ment, Kinematics, Statics and Dynamics, Molecular Physics, 

 Sound, Light, Heat, Magnetism, Electricity, Astronomy, Ap- 

 plied Mechanics — [as the Exhibition must be regarded as chiefly 

 referring to education, research, and other scientific purposes, it 

 must in this division consist principally of models, diagrams, 

 mechanical drawings, and small machines, illustrative of the 

 principles and progress of mechanical science, and of the appli- 

 cation of mechanics to the arts], — Chemistry Meteorology, 

 Geography, Geology and Mining, Mineralogy, Crystallo- 

 graphy, and Biology. 



Mr. Sullivan on Tuesday, in the Heuae of Commons, moved 

 with regard to the necessity for having a Museum of Science 

 and Art in Dublin. He, as well as the^other speakers, seems to 

 be ignorant of the fact that in addition to its educational staff 

 and appliances, the Royal College of Science in Dublin possesses 

 the germ of an admirable museum which formerly constituted the 

 Museum of Irish Industry. It seems probable that what is needed 

 is a removal of the specimens from the College to a suitable build- 

 ing ; probably an enlargement of the Royal Dublin Society would 

 be best, and the space thus gained in the College of cience 



