218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
its poor conducting qualities. This is the case with mineral wool, a 
sort of fibrous glass made out of the slag of blast furnaces. 
(6) When by reason of circumstances, such as the breaking of a 
water tube, etc., an insulating material is wet it should be able to dry 
out easily and regain its property of poor conduction. 
(7) The insulating substances should not be attractive to parasites, 
mice, rats, etc., nor afford a good culture ground for microbes. 
(8) The insulation material should be incombustible, or at least 
should not propagate combustion started at any point of the mass. A 
certain number of cork mixtures possess this property; for example, 
the mixture of cork and pitch. M. Briill has shown to the congress 
several different types of entirely fireproof cork mixtures. 
(9) When once placed in the packing which makes up the insu- 
lating mat, either inside or outside of the wall, the insulating ma- 
terial should not settle and thus produce continuous voids in the 
insulation. The different wood carbons included under the term 
charcoal are liable to this disadvantage, when they are used without 
special precautions. 
(10) The insulating materials should not attack the wood, iron, or 
masonry which comes in contact with them. 
(11) The insulating materials should be very easy to work and 
to apply to the walls of the storage chambers and should possess a 
certain resistance to bending or crushing. 
(12) The insulation should not lose its qualities with time. 
It is difficult enough to find an insulation that combines all these 
qualities. Cork either in granular form or agglomerated, however, 
is at present the most employed. M. Pasquay has informed the 
congress that silk waste protected by an impermeable envelope forms 
an excellent insulation. 
The knowledge of the coefficient of conduction of insulators is of 
great importance with regard to the thickness which the protecting 
linings must be to bring down the loss of cold within a certain limit. 
Different methods have been proposed for determining this. In 
one of these, the two faces of a plate of the substance are maintained 
constantly at different temperatures and the quantity of heat passing 
through the plate in a certain time determined. This may be ac- 
curately measured by weighing the amount of ice melted. This is 
the principle of the well-known physical method called the wall 
method. It may perhaps be remarked that those who have used 
this method have not taken precautions against the loss of heat at 
the edges of the experimental plates. They have not made use of 
the method of using a guard ring in the form originated by M. Berget. 
The other methods for measuring the conductibility are based on 
Forbes’ method. This consists of heating one of the extremities of 
a long slender bar of the material to be tested. When the system has 
