TEANSPAEENCT OF VAEIOUS SOLIDS. 
869 
permeability to the rays which excite chemical action. Ice (and water), as well as white 
fluor-spar, rival it ; and pure rock-salt approaches it very closely *. White topaz is a 
little inferior to the preceding bodies in diactinic capacity. 
Amongst the various compounds submitted to examination, the fluorides rank first in 
diactinic power ; then follow the chlorides of the metals of the alkaline earths. The 
hromides of the same metals appear to be less diactinic than the fluorides and chlorides, 
and this decline in power is still more marked in the case of the iodides. The short 
spectrum of these last-mentioned salts is interrupted by a well-marked absorption-band at 
a point beyond H, represented on the arbitrary scale at 10 3 '5, beyond which the spectrum 
is again faintly renewed to 113‘5, and then it terminates abruptly. The cyanides appear 
to be considerably diactinic ; but further experiments upon these salts, as well as upon 
the sulphocyanides, are desirable. Sulphuric, carbonic and horacic acids furnish salts with 
the alkalies and alkaline earths, which are also largely diactinic : the phosphates seem to 
be less so, and the arseniates still less. It is remarkable that though the sulphates are so 
diactinic, the sulphites are considerably less so, and the hyposulphites are more opaque 
than the sulphites. The hydrates of the alkaline earths are also transparent. It is very 
difiicult to obtain the alkaline hydrates perfectly pure ; but a solution of hydrate of soda 
and one of potash, furnished by precipitation of their respective sulphates by means of 
baryta, and concentrated in a silver dish, gave a very fair result in each case. 
The diactinic capacity of the tartrates and citrates is less than that of the carbonates. 
That of the acetates appears to be about the same as that of the tartrates ; but the results 
obtained with the acetates are somewhat uncertain, as it is difficult to procure these salts 
absolutely free from the empyreumatic products Avhich accompany the acid as it is usually 
prepared. The oxalates have a low diactinic power. 
But the group most remarkable for its absorptive action is formed by the nitrates. 
Nitric acid, whether dissolved in water or in combination with a metallic oxide, has a 
specific action in arresting the chemical rays : the more refrangible portion it transmits 
freely, and then intercepts the spectrum abruptly at the same point whatever base be 
united with the acid, provided the base be capable of forming diactinic salts. The 
chlorates, on the contrary, are strongly diactinic. 
13. From the observations above detailed, it appears that the following acids may be 
considered as possessing high diactinic capacity, — viz. the sulphuric, hydrochloric, hy- 
drofluoric, chloric, carbonic, and boracic acids. Inferior to these are hydrobromic, 
phosphoric, arsenic, tartaric, citric, acetic and oxalic, hydriodic, sulphurous and hypo- 
sulphurous acids, whilst nitric acid is still less diactinic. Chromic acid arrests all the 
chemical rays ; and the presence of a tinge of yellow or green colour in any compound 
is immediately apparent in a great reduction in the amount of its diactinic power. 
14. Among the bases, potash, soda, ammonia, baryta, lime, strontia, magnesia, and 
alumina are eminently diactinic. Oxides of zinc, mercury, and lead approach them in 
* A specimen of sea-water whicli had been standing for some months in my laboratory, furnished a re.sul 
identical with that obtained by using a strong solution of pure chloride of sodium. 
MDCCCLXII. 6 C 
