( 659 ) 
The said yellow salt has been investigated by Brooks as to its 
behaviour at higher temperatures. He found that it had lost hardly 
any weight at 220°, and not until 260° brown vapours were evolved. 
This behaviour, I think, explains the formation of that salt on 
heating Hg, (NO3)s. At the then reigning temperature it is formed 
because it can then exist and at the same time volatilise. For the 
greater portion of the salt the opportunity for sublimation does not 
exist and the increased temperature decomposes it completely. 
It is still more interesting to know that this salt is also formed 
by heating mercuric nitrate. Absolutely mercurous-free He (NOs), 
heated in the above-mentioned apparatus yields the same yellow 
sublimate. It has, therefore, been formed in a strongly oxidising 
surrounding containing oxygen and NOs. If pure mercuric nitrate 
is heated in a porcelain dish until the decomposition is fairly started, 
the still undecomposed white crust which covers the red powder 
of nitrate-containing HgO (free from Hg,O) will be found covered 
on the inner surface with a yellow substance which on analysis is 
found to contain mereurous-mercurie nitrate. The white crust, however, 
does not contain any mercurous compound. 
The formation of this compound, particularly during the decom- 
position of mercuric nitrate, leads me to suppose that during the 
heating complex molecules of 1Ig4 (NO3)s or (in the case of mercurous 
nitrate) complex molecules of He, (NO), are subjected to decom- 
position. The latter then yield (Hg,) Hes (NO), with elimination 
of 2 NOs, while the former do so with elimination of 6 NO, and 2 Os. 
I must also state that I have heated anhydrous Hg, (NOs). in 
vacuum tubes at different temperatures and have always noticed 
that the salt turned yellow and was converted to a larger or smaller 
extent into the said orthonitrate with evolution of a brown gas 
which behaved in every respect like NO». 
Physics. — W. H. Kersom. “Contributions to the knowledge of 
VAN DER WAALS’ w-surface.” VI. The increase of pressure 
at condensation of a substance with small admixtures.” (Com- 
munication NP. 79 from the Physical Laboratory at Leiden, 
by Prof. H. KAMERLINGH ONNES.) 
(Communicated in the meeting of March 29, 1902). 
§ 1. In Communication N°. 75 I have expressed by the two constants 
l (aT or 1 /dpsk 
= — (=) and nn ( Pe ) 
Py dE Ard Pk de /zy—0 
the corrections by means of which we can derive the critical data 
