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reaching the cooler water. When the water gets thoroughly heated, 

 the bul)bles of steam reach the surface, and finally pass out of the 

 mouth of the flask. There they are partially condensed into a cloud or 

 fog, which can be seen. 



There are many points of difficulty in this with the children, from 

 former notions picked up here and there. It is hard to understand that 

 the steam is invisible, as are many of the vapors in the preceding les- 

 sons (of gasoline, ether, etc.). But these very difficulties give oppor- 

 tunity for questions, which they may discuss with each other, for the 

 settling of which they may devise experiments. Do not be in too great 

 a hurry to have it all taught. We may remember that what we wish of 

 our material is, that it may give iust those questions which it is possible 

 for the children to work on. We should give them a chance to work 

 at these questions when we find them. For those pupils who are ready 

 for it, the thermometer may be used, and the gradual rise of tempera- 

 ture observed until the water boils, when it will be seen that it stands 

 at 212° Fahrenheit, '.whether in the boiling water, or the steam just 

 above it (not in the fog formed outside). Alcohol may be used to show 

 that other liquids have a lower boiling point. 



CONDENSATION. 



The phenomena of condensation will constantly come up in the fore- 

 going experiments, and is then, of course, to be noted. This is es- 

 pecially so with camphor, iodine, and water. Many experiments may 

 be devised to further illustrate the results of evaporation and condensa- 

 tion of water. It must be seen that such high degrees of heat as for 

 boiling is not needed for evaporation. 



A glass vessel, partly filled with water, and the mouth closed, will 

 show water constantly on its inner surface, coming from the condensed 

 vapor arising from the water. 



The condensation of water on the cold surface of a plate of glass, ex- 

 posed to the breath, or to the surface of the skin, or the under surface 

 of a leaf, or over the flame of a lamp or candle, will show that there is 

 vapor of water coming from all these sources. 



Water may be distilled from a flask , by connecting it by a tube to 

 another vessel. The water boiled in the flask will pass as steam into 

 the tube, which is kept cool by moistening it. From the tube the water 

 drops into a receiving vessel. 



It may be said, in passing, that it is now taught that in the formation 

 of fog and clouds, each particle of water of the fog or cloud has conden- 

 sed upon a particle of dust in the air, and if there were no particles of 

 dust there could be no fog, clouds, or rain. By "dust" is meant any 

 small object floating in the air, such as go to make up what we com- 

 monly speak of as dust and smoke. 



The application 3fl4 illustration of tU§ ^bove lessons wUl occur to 

 6SQU 9m, 



