Chemistry and Phystes, 401 
10. Photographs of Nebula,—M. J. JANssEN calls attention to 
the effect of short and long pr pris upon the negatives which 
are obtained. Photographs of the same nebula will not agree 
“unless the same conditions of rence are narrowly observed. In 
proof of this the photographs of the solar corona taken at Siam in 
the eclipse of 1875 are referred to: the nebulosity, so to speak, of 
the corona gave different impressions upon sensitive plates which 
were exposed during times expressed by the numbers 1, 2, 4, 8: 
and it must be inferred ate the changes in the height of the corona 
are to be attributed to the times of exposure, hers of to actual 
variations in the extent of the phenomenon. is then indispens- 
able that the photographs of nebule should accompanied by 
some evidence ee the conditions under near he they are taken, in 
Dingle poly. fy ola p. 426, pe ‘| is much Pa 
than the Gim am pump which was used b sae gs and is 
not so liable: Po break. It allows the vessel which i e 
exhausted to be connected to the pump without the denna 
tion of sto cks or metallic joints. N niga ee: a continuous 
glass tube Ni ag with the p cury packing 
is employe mportance of heating the bs hioks is to be 
exhausted i in spe to drive off the layer of air which adheres to 
the sides of the vessel is insisted upon en has obtained 
an exhaustion of 0:000012™", while Crookes has obtained only 
0-000 0046™". Besides heating the vacuum tube to expel the air 
ary: 
Th avor to detect the passage of aqueous vapor 
aioe the sides of the vacuum tube by spectrum analysis gave 
a negative result. gsi Pie der Physik und Chemie, No. 8, 1881. 
12. Absorption a the Sun’s rays by the Carbonic Acts of "the 
Atnoaphers —H. ‘st Lecuer, by means of observations with 
SE ek Hart atin apparatus in connection with observations with 
a pyrheliometer, arrives at the conclusion that the amount of car- 
bonie acid which has been proved to exist in the air is sufficient 
