The nomenclature of the rubber products which are to be discussed in this paper has not been too exact. There are patents entitled froth rubber, expanded rubber, and cellular rubber, all of which refer to the same product. One patent named cellular rubber describes a product which is similar to that of another patent called microporous rubber. Sponge rubber, porous rubber, foam rubber, and other names have been used rather loosely, and manufacturers themselves have not been in complete agreement as to the correct names for some of these products. In most groups of products there is considerable similarity between the different members of the group, but this is not true in this family of cellular rubbers; for example, a microporous hard rubber battery separator is quite dissimilar to a latex foam rubber cushion but they do have one thing in common—they are both cellular, that is, consisting of cells, which are defined as small hollow receptacles. The first problem then is to classify and define the various products included in the cellular rubber family. In this paper the term rubber has been used to include both natural rubber and the synthetic types, except where one or the other is specifically mentioned.