.( 707 ) 
a material that does not reverberate sound; for if this is neglected, 
not only the involuntary sounds that are made by us, will have a 
disturbing influence, but we shall also be hindered by the small 
remainder of sound that might still be left on account of income 
pleteness in the construction; the resonance of the space that is shut 
off will itself seize definite parts of the small quantity of noise that 
arises or penetrates into it and make them audible in a higher 
degree. 
2. The isolation must be brought about by a double wall, with 
interstices of air of such a trifling thickness that resonance of audible 
tones is quite out of the question and moreover no other contact is 
left between the two walls than of a few narrow lead-contacts. 
3. The isolation of the outer wall of the building and of its 
bottom has to be as complete as possible: the first isolation has to 
take place through a purposely constructed secondary apartment. 
The first condition is fulfilled in our laboratory by means of a 
covering of horsehair some centimeters thick (trichopiese), as it is 
used in telephone-cells. Thanks are due to Dr. Biltris of Gent for 
making me acquainted with this material, which, moreover, procures 
an excellent isolation of sound. 
The second condition is satisfied at Utrecht by making use, in 
fastening the trichopiese, of a wall of porous stone and by con¬ 
structing outside it a second wall, consisting of corkstone of German 
manufacture. Plates of peatmoss from Klazienaveen in the province 
of Drente would have answered the purpose even better. 
The third condition requires the exclusive use of lead-contacts. Espe¬ 
cially the bottom has to be well provided for. At Utrecht faults have 
been made in this respect, which could only partly be made up for 
by the subsequent addition of an extra-covering. 
Taking the above-named chief conditions for granted, we shall 
have to answer the question, whether an apartment free from sound 
will have to be constructed underground, on a level with the ground 
or on a higher floor. My answer is decidedly on a higher floor, for 
the conduction of the sound coming from the bottom is the obstacle 
which it is most difficult to overcome. An efficient isolation of the 
bottom can much more easily be brought about on a higher floor 
than on a foundation. In the first case the only thing one has to do 
is to provide lead-contacts with the stone beams, which in their turn 
are not directly connected with the bottom, whilst in the second 
case, under the most favourable circumstances, short columns con- 
