SCIENTIFIC SUMMARY. 
345 
seems likely to be conducive to discovery in this department of physics, has 
been devised by Mr. H. C. Sorby. Mr. Sorby proposes that it shall be used 
as a scale in all descriptions of spectra as seen by the micro-spectroscope. 
The following is Mr. Sorby’ s description of the new scale. It is an inter- 
ference spectrum, produced by a plate of quartz -043 inch thick, cut parallel 
to the principal axis of the crystal, and placed between two Nicol’s prisms. 
In this the whole visible space is divided by dark bands into twelve regular 
divisions, having in all parts the same relation to the physical properties of 
the light. These are counted from the red end towards the blue, their 
centres being reckoned as 1, 2, 3, &c. and the thickness of the plate is so 
adjusted that the sodium line exactly corresponds to 3 a. The intensity of 
the absorption is expressed by the following types : — 
Not at all shaded 
Very slightly shaded 
Decidedly shaded 
More shaded 
Strongly shaded, but so 
that a trace of colour is 
still seen 
Still darker 
Nearly black 
Blank space 
. . Dots trith wide spaces 
. . . Dots closer together 
. . . Very close dots 
Three hyphens close 
— Single dash 
Double dash 
Except when specially requisite, only the symbols . . . — — are 
employed for the sake of simplicity, and then as signs of the relative rather 
than of the absolute amount of absorption, and it is assumed that there 
is a gradual shading off from one tint to the other, unless the contrary is 
expressed. — Chemical New s, May 3. 
The Contractions and Dilatations of Iodide of Silver. — At a meeting of the 
French Academy on April 15, M. Fizeau returned to this subject. He 
found that with iodide of silver, whether in its amorphous or crystalline 
state, the action of heat established by him is reversed ; its dilatation is 
negative, contracting instead of expanding on the increase of temperature. 
But this negative dilatation is not quite the same in the amorphous state, 
in the state of compressed precipitate, as in the crystalline state ; 
— 000000137 in the first case, —0-00000139 in the second. 
The Determination of the Density of Ozone. — A note on this subject was com- 
municated to the Academy of Science by Mr. Soret, of Geneva. Experi- 
ments by means of absorption lead to the conclusion that the density of ozone is 
one and a half times that of oxygen. lie applied Graham’s law of diffusion 
— viz. that the diffusion takes place in the inverse proportion of the square 
of the density. He then diffused two mixtures — one of oxygen and chlorine ; 
the other of oxygen and ozone. Thus compared, the density of ozone to 
that of chlorine or oxygen was found to be 1 : 5. 
Development of Ozone during the present year. — A correspondent of the 
Chemical News gives an acconnt of the development of ozone since the first of 
January. In January there were two well-marked periods of scarcely a 
trace of ozone — namely, from the 1st to the 5th, culminating on the 4th, 
when the test was colourless, and from the 10th to the 19th, culminating 
on the 13th, 14th, and 18th. Both these periods were followed by 
