"The impermeability of a pipe may be tested by tying a piece of bladder over one end, reversing it, and filling with water. If it is not perfectly impervious, the water will begin to ooze through the pores" of the pipe.
The absorption of water, ascertained by weighing a dry pipe, immersing it for twenty-four hours, wiping dry, and reweighing, was found by Mr. Baldwin Latham to vary from '429 to 6-8 9 per cent of the weight of the dry pipe.
The power to resist chemical action may be tested by pulverising a piece of the pipe and boiling it in hydrochloric acid, washing on filter, and noting loss in weight. Dr. Millar has shown that in stoneware pipes there should be no loss.
This is ensured by casting upon the spigot and in the socket of each pipe, rings of a durable material (a mixture of coal-tar, sulphur, and ground pipes), which, when put together, fit mechanically, so as to form a water-tight joint without the aid of cement.