440 Mr. Davies’s Supplementary Note on the 
of course). For the latter, Dutch gold-leaf may be substituted 
with advantage. 
The glass combustion tube is best filled with the oxide of 
copper, &c. and subject of analysis, before the precipitation of 
the copper upon it, and the tube is best drawn out to a neck 
at the end next the kali apparatus, as well as at the remote one, 
the former being done immediately after the filling ; the latter 
neck is opened on commencing the combustion, and the union 
with the train of absorbent vessels is made by cork in the usual 
way, but in inverse order, that is, the first cork is not inserted 
into the combustion tube, but placed pon the drawn-out neck, 
thus 
The whole tube from A to B being covered with copper, the 
passage for the gases is ensured by a sharp blow or two ona 
table of the combustion tube as usual. 
In the methods proposed by Prof. Bunsen of Marburg 
some time ago, chiefly for the determination of nitrogen by 
combustion in a hermetically sealed tube, he imbedded the 
combustion tube in a mould of plaster of Paris to prevent the 
elastic gases evolved from bursting the tube. The process 
was difficult and uncertain, but the application of the method 
above described gives as much facility to the performance of 
organic analysis by his method as by any other. 
This mode of precipitating copper upon glass is also sus- 
ceptible of many other useful applications in the arrangement 
of chemical apparatus when heat or pressure is concerned. 
I have the honour to be, Sir, 
Your obedient Servant, 
Dublin. Rospert Matuet, Ph.D. 
LXXV. Supplementary Note on Brett’s determination of the 
Foci of a Conic Section. By 'T. 8. Davies, Esq., £L.S. 
Lond. and Ed., F.S.A., Royal Military Academy*. 
REFERRED to the paper of M. Brett when discussing 
a more general problem in the Philosophical Magazine 
for January last, and I stated that the author had not solved 
his resulting equations, even as adapted to the confined case 
which he proposed to consider. The most general form of 
the problem, however, when confined to the foci, without 
* Communicated by the Author. 
