( 708 ) 
sisting of many strata can be ipade use of, which, however, have a 
constant direct communication with the ground. 
As to the different tones, the most difficult thing appears to be to 
keep away the low tones. Inaudible vibrations of very slow perio¬ 
dicity are even not at all excluded in our camera silenta, so that a 
sensitive microphone, conducted to a gold-thread string-galvanometer 
does not appear to subside, not even when at a complete adaptation 
of the organ of hearing not a trace of sound is to be observed. 
(This does not disturb acoustically, but a somewhat faster periodicity 
would have been a hindrance). 
Besides an apartment free from sound ought to have porous walls, 
for if perfectly impermeable walls are chosen, it will appear that in 
case of long experiments a ventilation is necessary, which in its turn 
would require the supply of ventilation-channels, consequently of 
sound-leaks. For double-door and double-window (the latter in my 
opinion hygienically indispensable) as a matter of course apparatus 
are wanted which require much care and a lasting control. When 
acoustic experiments are made, the supply of sound should come 
from sound-sources placed outside the apartment, right through a 
leaden stopper, that the principle that the two walls of the double 
wall should have none but a lead-contact, is not discounted l ). Electric 
light, telephone, supply of air for organ-pipes and sirens through a 
narrow leaden tube and the necessary conducting-wire to the galvano¬ 
meter offer no technical difficulties. 
An accidental additional advantage of an acoustic apartment with 
a double wall, double door and double window, duly separated from 
he outer-walls of the building by means of by-apartments, is this, 
hat it forms a calorimeter. The camera silenta at Utrecht remains 
without an inhabitant of a constant temperature to within 2 deci- 
grades. By covering the trichopiese-walls with some meters ofextre- 
mely fine tresswire (0,1 mm.), a bolometer may be made with a 
"“T b " d 8 e and galvanometer placed in a by-apartment, by 
.. , 0me K er k . 1 e ™* °*' ,em perature that the space undergoes 
” ay be measured - The production of heat which 
irr ‘ S , d ned k empirically (d’Aksonval). As a respiration- 
SisT hTT' * he S ° Und - free a P artment ** be «™d. 
un it kTT ^ Ca " Se the Wa “ 8 are P oro,,s - and » ^ given 
A nlh r ger fr ° m S0Und f0r Io nger experiments. 
z mvestigations may take place in the camera silenta. 
'“I" Sl0PPW . S arC 5 cm ' thick and possess a central bore, at its narrowest 
pom. bem ? 0.4 cm, wide; comp. Qode*. Ph, s i ol . Lab . Utwhl (5 , VJ . p . m 
