44 



SCIENCE. 



[Vol. I., No. 2. 



and its superior tension acts upon both bodies toward 

 a common centre of concavity." 



We do not think physicists generally will admit 

 that a liquid film tends to draw a solid, to which it is 

 attached toward the centre of concavity of the film. 

 Indeed, if this were so, tlie tendency of a column of 

 water raised between two floating bodies by surface 

 tension would be to lift those bodies : similarly a col- 

 umn of liquid sustained in a fine tube would tend to 

 lift the tube. This action, however, is denied both 

 by theory and experiment. In fact, unless we have 

 misunderstood Prof. Leconte's language and dia- 

 grams, his article will not do all that it was intended 

 to do, toward removing the difiiculties in the way of a 

 student beginning the study of capillary phenomena. 

 * — {Amer.journ- sc, Dec, 18S2.) E. H. H. [92 



Rigidity of the earth. — G. H. Darwin discusses 

 the long-period tides — the lunar fortnightly declina- 

 tional and the lunar monthly elliptic — from .33 years' 

 observations in England, France, and India, and finds 

 that they are reduced to 0.7 of their theoretic height. 

 There should be no reduction on a rigid earth, aiid 

 no ocean tides on a liquid earth : as the actual effect 

 of the earth's yielding to the moon's attraction is only 

 0.3 of the difference between these extreme effects, 

 the earth is considered at least as rigid as steel. — 

 (iVatore, Nov. 2, 1882.) w. M. D. [93 



Optics. 



Molecular refraction. — In an investigation on 

 the refractive powers of carbonic ether and its sulphur 

 substitution products, E. Wiedemann finds that the 

 atomic refraction of sulphur depends upon its place 

 in the molecule as does that of oxygen. — ( Wied. ann., 

 Dec, 1882.) c. s. h. [94 



Dispersion formulas. — A. Wiiliner shows that in 

 a large number of colorless substances, in which case 

 the absorption constant may be regarded as zero, two 

 of the constants in Helmholtz' dispersion formula 

 are sensibly equal, and the formula reduces to one of 

 two constants, which is then equivalent to that of 

 Lommel. The same was found to hold true of an 

 alcoholic solution of alizarine, as also of an aqueous 

 solution of anion io-sulphate of copper. — ( Wied. ann., 

 Dec, 1SS2.) c. s. H. [95 



Diffraction. — A series for the calcvdation of Fres- 

 uel's integrals, and a table of values, are given by A. 

 Lindstedt. — ( [Vied, ann., Dec, 1882.) c. s. H. [96 

 iPhotometi^.) 



Photometric observations of the transit of 

 Venus. — Professor E. C. Pickering has made some 

 comparisons of the brilliancy of the sun, of Venus, 

 and of the region in the immediate vicinity of the 

 limb of the sun, whereby the photometric illumina- 

 tion of that portion of the corona may be determined. 

 An ordinary double-image-prism photometer with a 

 few slight modifications, attached to the tail-piece of 

 the 1.5-inch equatorial, was employed for the observa- 

 tions. Calling the light of the sun 100, the mean of 

 thirty-two settings taken between 1 h. 07 min., and 

 1 h. 30 min., Cambridge mean time, gave: Venus 1.8, 

 and the sky 8.8. The mean of twenty-four settings 

 taken between 2 h. 48 min. and 2 h. 5-1 min. gave: 

 Venus 1.4, and tlie sky (5.2. The mean of all gave: 

 Venus 1.6, and the sky 7.5 or 4.7 times as bright 

 as Venus. [According to this, the light of this por- 

 tion of the corona would seem to be about 3.7 times 

 as brilliant as the light reflected by that portion of 

 our atmosphere lying between us and the sun.] — 

 w. H. P. ' [97 



Heat. 



Thermal conductivity of rocks. — A novel 

 method has been employed by M. Thoulet for the 



determination of the thermal conductivity of minerals 

 and rocks. Instead of measuring the temperatures 

 at different distances from the source of heat, meas- 

 urements are taken of the time required for the pas- 

 sage of a certain quantity of heat through a section of 

 known thickness. The ' thermal resistance ' is de- 

 fined as the time required for the passage of a definite 

 quantity from a source at 100° C. through a thickness 

 of 0.01 mm. The thermal resistance is consequently 

 inversely proportional to the thermal conductivity. 

 Glass and iron have already been experimented on, 

 and the method appears to give very accurate results. 

 — {Ann. chim. phys., (5). xxvi. 261.) c. B. P. [98 



Heat of combination a function of atomic 

 ■weight. — Mr. Laurie shows, that if the atomic 

 weiglits of elements are taken as abscissae, and their 

 atomic heats of combination with chlorine, bromine, 

 or iodine, as ordinates of a curve, the heats of com- 

 bination will be seen to be a periodic function of the 

 atomic weights. — {Phil, may., Jan., 1883.) c. B, P. 



[99 

 Electricity. 



Electrical resistance of selenium cells. — In a 

 communication to the Physical society of London in 

 June, 1881, Dr. .lames Moser urged that the decrease 

 in electrical resistance observed in a 'selenium cell' 

 when acted upon by a beam of light, is due to heat- 

 ing, which by expanding the selenium makes it press 

 more firmly against the metallic electrodes of the 

 cell, thus establishing better connection. The fact 

 that luminous are more effective than obscure rays 

 in producing the observed change. Dr. Moser sought 

 to explain as a result of selective absorption. Sir. 

 Shelford Bidwell undertook to put Dr. Moser' s theory 

 to the proof by heating selenium cells to known tem- 

 peratures in the dark, and observing the consequent 

 change in electrical resistance. It appears from his 

 experiments, that below a certain temperature, which 

 is difierent for each cell, heating increases the elec- 

 trical resistance of the cell; that above this tempera- 

 ture heating decreases the resistance, the temperature 

 of maximum resistance being usually somewhat 

 above ordinary temjierature, but in one case being 

 13° C. Mr. Bidwell concludes, however, that the 

 action of the luminous rays upon the cell cannot be 

 explained by their heating effect alone; for he finds 

 that whereas a moment's exposui'e to direct sunlight, 

 though causing a great fall of electrical resistance, 

 yet lieats the selenium to a hardly perceptible extent, 

 an equal decrease of resistance 'caused by heating in 

 the dark could be produced only by making the cell 

 too hot to handle. Mr. Bidwell concludes from his 

 experiments that the action of the carbon photophone 

 is to be explained by the heating alone. — (Phil, 

 mag., Jan., 1883.) E. H. H. [100 



EBTGIKTEEIlIBrG. 

 Practical test of the safety of bridges.' — It is 

 well known among engineers, that, with good iron 

 properly used, our bridges may he relied upon for an 

 indefinite length of service. The best practice never 

 loads a structure with more than from one-fourth to 

 one-sixth of the weight that would break it down. 

 Any load put upon a'piece of iron will stretch it to 

 a slight extent. Upon removing the load, the iron 

 should regain its original form. This it will do if it 

 has not been overstrained. A very simple and effec- 

 tive piece of mechanism has been for some time in 

 use at the East-Eivei' suspension-bridge at Bi'ooklyn 

 for determining the precise effect of any load upon 

 any part of the structure. A bar ten or twelve feet 

 long is attached to any member of tlie bridge in such 

 a manner that any increase in the length of such 



