248 Mr. J. J. Waterston on Chemical Notation in 



scale, to the divisions of which they are referred as abscissas. In 

 order to render these drawings, which have reference to our in- 

 strument, applicable to observations upon the scale of any other 

 apparatus, which we may call B, it is only necessary to prepare 

 a reduced scale which is laid upon the several drawings and used 

 in place of the divided scale given on the Plate. The lines 

 marked at the bottom of Plate V. serve for the preparation of 

 this new scale : these lines denote the distances between the 

 lines Ka, Li a, Na, Tl, Sr S, Rb a, and K/3, measured according 

 to the scale of our instrument. The position of each of these 

 lines is determined by the edge of the line, which does not change 

 its place on altering the breadth of the slit. The position of these 

 same lines is read off on the scale of the instrument B, and the cor- 

 responding number written under each. A series of fixed points 

 on the scale is thus obtained, and the complete divisions for the 

 scale of the instrument B are got by interpolating the values of 

 the portions of the scale situated between the fixed points. The 

 sodium line is then inserted in this scale, which is pasted upon 

 a straight-edge, and the divisions numbered in tens and fives. 

 If this measure be now laid upon any one of the drawings so 

 that the sodium line on the measure coincides with the division 

 50 on the drawing, the scale on the measure will give the position 

 of all the lines in the particular spectrum exactly as they are 

 seen in the photographic scale of the instrument B. When the 

 position of the line under observation has in this way been ascer- 

 tained, it is easy to assure oneself of its exact identity by means 

 of the small prism on the slit. 

 Heidelberg, April 26, 1863. 



XXXV. On Chemical Notation in conformity with theDynamical 

 Theory of Heat and Gases. By J. J. Waterston, Esq.* 



IN a lecture " On the Molecule of Water," lately delivered at 

 the Royal Institution, Albemarle Street, by Mr. Odling, 

 Secretary to the Chemical Society f, the question as to the mole- 

 cular weight of water was discussed with much detail, and argu- 

 ments adduced to show that the correct number is 18, represented 

 by the formula H 2 0, and that it cannot be 9, as might be in- 

 ferred from the specific gravity of the vapour, following the idea 

 that equal volumes of bodies in the gaseous state contain an equal 

 number of atoms. 



The dynamical theory of heat and gases imperatively requires 

 us to believe that in equal volumes of gases and vapours in sequi- 



* Communicated by the Author. 



t A complete report of the lecture is given in ( The Engineer' for June 26. 



