What should be the relative difference in temperature between the hot and cold particles of water to create a practical circulation?

Seven degrees.

Explain the bursting of lead pipe by frost.

Water expands from 39.2 degrees F. until it reaches the boiling point. Therefore water is at its greatest density at 39.2 degrees F. It also expands in cooling until it reaches 32 degrees F. The water will then remain at 32 degrees, and will gradually solidify until all of the latent heat is extracted, when we have ice, the water having expanded about 9 per cent. during solidification. Below this temperature the ice contracts in volume. This being the fact, in time it releases its hold on the inner surface of the pipe, and the water still remaining in a liquid state, works its way into this space. This again expands as before, and so on, until finally the pipe, which can no longer stand the strain, hursts.

Why does one frosted pipe burst while another does not?

For the reason that the ice in one is allowed to expand, while in the other it is not. For example, we will suppose there is a section of exposed pipe a few inches in length, and the water in this exposed section freezes. The result is that it expands lengthwise and no damage is done. Now suppose there are two exposed sections a foot apart. Both sections being frozen, there is no chance for the water between to expand lengthwise as it freezes, and the force is necessarily exerted against the sides of the pipe. This would hold good where an entire length of pipe was frozen. Some portions of the water solidifying quicker than the others, prevents any expansion lengthwise of the pipe, and in consequence, the expansive force is exerted sidewise, and results in the bursting of the pipe.

What is the effect of carbonic acid gas or sewer gas on lead and iron?

This gas acts chemically on both metals, especially on lead, thereby destroying them after a time.

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