Ang. 12, 1 875 J 



NATURE 



301 



the pressures at which carbonic acid gts liquefies. It gave, 

 indeed, the pressures exercised by the liijuid when contained in 

 large quantity in a Thilorier's reservoir ; but these pressures are 

 always considerably in excess of the true pressures in conse- 

 quence of the unavoidable presence of a small quantity of com- 

 pressed air, although the greatest precautions may have been 

 taken in filling the apparatus. Even ^^^ part of air will exer- 

 cise a serious disturbing influence when the reservoir contains a 

 notable quantity of liquid. 



Law of Boyle.— ThQ large deviations in the case of carbonic 

 acid at high pressures from this law appeared distinctly from 

 several of the results given in my former paper. I have now 

 finished a long series of experiments on its compressibility at the 

 respective temperatures of 6°7, 63°7, and 100° Centigrade. The 

 two latter temperatures were obtained by passing the vapours of 

 pyroxylic spirit (methyl alcohol) and of water into the rectan- 

 gular case with plate-glass sides, in which the tube containing the 

 carbonic acid" is placed. The temperature of the vapour of 

 the pyroxylic spirit was observed by an accurate thermometer, 

 whose indications were corrected for the unequal expansion 

 of the mercury ; while that of the vapour of water was 

 deduced from the pressure as given by the height of the 

 barometer and a water-gauge attached to the apparatus. At 

 the lower temperature (6'^ 7) the range of pressure which 

 could be applied was limited by the occurrence of liquefac- 

 tion ; but at the higher temperatures, which were considerably 

 above the critical point of carbonic acid, there was no limit of 

 this kind, and the pressures were carried as far as 223 atmo- 

 spheres. I have only given a few of the results ; but they will 

 be sufficient to show the general effects of the pressure. In the 

 following Tables p designates the pressure in atmospheres as 

 given by the air-manometer, /' the temperature of the carbonic 

 acid, » the ratio of the volume of the carbonic acid under one 

 atmosphere and at the temperature /* to its volume under the 

 pressure /' and at the same temperature, and 6 the volume to 

 which one volume of carbonic acid gas measured at 0° and 760 

 millimetres is reduced at the pressure/ and temperature f : — 



Carbonic Acid at 6° 7. 



13-22 



20-I0 

 24-81 



3 1 06 

 40-11 



/• 



at. 

 16-96 



6-90 

 679 

 673 

 662 



6-59 



I 

 i4-3"6 



I 

 23-01 



I 



25r-6o 



I 

 39'57 



58^40 



Carbonic Acid at 63° -7. 



f. *. 



63-97 



54-33 63-57 



106-88 63-75 



145*54 63-70 



222*92 63-82 



17-85 



66-06 

 I 



185-9 



I 



"32r3 



I 



at. o 



i6-8o 100-38 



53-81 100-33 



105-69 100-37 



145-44 99-46 



223-57 99-44 



446-9 



Carbonic Acid at 100°. 



I 



17-33 



I 



60 -22 



I 



137-1 

 I 



2r8-'9 

 I 



380-9 



007143 

 0-04456 

 0-03462 

 0-02589 

 001754 



e. 

 006931 

 001871 

 0-00665 

 0-C0378 

 000277 



9. 

 0-07914 

 002278 

 o-oiooi 

 0-00625 

 o 00359 



These results fully confirm the conclusions which I formerly 

 deduced from the behaviour of carbonic acid at 48°, viz. that 

 while the curve representing its volume under different pressures 

 approximates more nearly to that of a perfect gas as the tempe- 

 rature is higher, the contraction is nevertheless greater than it 

 would be if the law of Boyle held good, at least for any tempe- 

 rature at which experiments have yet been made. From the 

 foregoing experiments it appears that at 63°-7 carbonic acid gas, 

 under a pressure of 223 atmospheres, is reduced to ^iy of its 

 volume under on« atmosphere, or to less than one half the 

 volume it ought to occupy if it were a perfect gas and contracted 

 in conformity with Boyle's law. Even at 100° the contraction 

 under the same pressure amounts to ,|x part of the whole. 

 From these observations we may infer by analogy that the 

 critical points of the greater number of the gases not hitherto 

 liquefied are probably far below the lowest temperatures hitherto 

 attained, and that they are not likely to be seen, either as liquids 

 or solids, till much lower temperatures even than those produced 

 by liquid nitrous oxide are reached. 



(To be continued.) 



NEIV METHOD OF OBTAINING ISOTHER- 



MALS ON THE SOLAR DISC* 

 /^N June 5, 1875, I devised a method for obtaining the iso- 

 ^^ thermals on the solar disc. As this process may create an 

 entirely new branch of solar physics, I deem it proper that I 

 should give a short account of it in order to establish my claim 

 as its discoverer. 



In the American Journal, July 1872, I first showed how one 

 can, with great precision, trace the progress and determine the 

 boundary of a wave of conducted heat in crystals, by coating 

 sections of these bodies with Meusel's double iodide of copper 

 and mercury, and observing the blackening of the iodide where 

 the wave of conducted heat reaches 70° C. If we cause the 

 image of the sun to fall upon the smoked surface of thin paper, 

 while the other side of the paper is coated with a film of the 

 iodide, we may work on the solar disc as we formerly did on the 

 crystal sections. 



The method of proceeding is as follows : beginning with an 

 aperture of object-glass which does not give sufficient heat in any 

 part of the solar image to blacken the iodide, I gradually in- 

 crease the aperture until I have obtained that area or blackened 

 iodide which is the smallest that can be produced with a well- 

 defined contour. This surface of blackened iodide I call the area 

 of blackened temperature. On exposing more aperture of object- 

 glass, the surface of blackened iodide extends and a new area is 

 formed bounded by a well-defined isothermal line. On again 

 increasing the aperture another increase of blackened surface is 

 produced with another isothermal contour ; and on continuing 

 this process I have obtained maps of the isothermals of the solar 

 image. By exposing tor about twenty minutes the surface of 

 iodide to the action of the heat inclosed in an isothermal, I have 

 obtained thermographs of the above areas ; which are sufficiently 

 permanent to allow one to trace accurately their isothermal con- 

 tours. There are other substances, however, which are more 

 suitable than the iodide for the production of permanent thermo- 

 graphs. 



The contours of the successively blackened areas on the iodide 

 are isothermals, whose successive thermometric values are in- 

 versely as the successively increasing areas of aperture of object 

 glass which respectively produced them. 



As far as the few observations have any weight, the following 

 appear to be the discoveries already made of this new method, 

 (i) There exists on the solar image an area of sensibly uniform 

 temperature and of maximum intensity. (2) This area of maxi- 

 mum temperature is of variable size. (3) This area of maximum 

 temperature has a motion on the solar image. (4) The area of 

 maximum temperature is surrounded by well-defined isothermals 

 marking successive gradations of temperature. (5) The general 

 motions of translation and of rotation of these isothermals appear 

 to follow the motions of the area of maximum temperature which 

 they inclose ; but both central area and isothermals have inde- 

 pendent motions of their own. 



On projecting the enlarged image of a sun-spot on the 

 blackened surface and then bringing a hot-water box, coated 

 with lamp-black, near the other side of the paper, one may 



• The discovery of a method of obtaining Thermographs of the Isothermal 

 Lines of the Solar Disc, by Alfred M. Mayer in Silliman's ^ w^r/Wj« Joitrtial 

 for July. 



