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excited interest through its being found in the sun and stars. For some time after the study of the sun's spectrum had been commenced, it was known that certain well-marked lines in it were not produced by any substance then known. But continued research led to the discovery that this substance existed in a Norwegian mineral, cleveite, and perhaps elsewhere on the earth. From its existence on the sun it was called helium. Its spectrum was no sooner made known than it was found that helium existed in many stars which are, for that reason, called "helium stars."

In many cases some idea can be obtained of the density of a star, or, in ordinary language, of its specific gravity. It is very remarkable that, in nearly all such cases the density is found to be far less than that of our ordinary solid or liquid substances; frequently no greater than that of air, sometimes even less. In this respect our sun, although its density is so small, seems to be an exception, and it is likely that only a very small proportion of the stars are as dense as the sun. This affords one proof of the high temperature of these bodies, which must be such that all liquid or solid substances exposed to it would boil away as water boils when put on a fire, thus changing its substance into a vapour. We have reason to believe that the stars are for the most part masses of this intensely hot vapour, surrounded perhaps by a somewhat colder surface. Possibly many of the stars