76 JUNIOR GRADE SCIENCE 



similar to the simple instrument already described, but the top is sealed 

 up, and divisions or graduations are marked upon it, so that the height 

 of the mercury in the tube can be seen easily. These divisions are called 

 degrees (Fig. 61). 



Feeling of heat and cold. Some people feel cold at the same time 

 that others feel warm. It is easy to understand that the sense of 

 feeling cannot be depended upon to tell us accurately whether the 

 air or any substance is hot or cold. Some instrument is needed which 

 does not depend upon feeling, and cannot be deceived in the way that 

 the senses can. Such an instrument is called a thermometer, and it 

 is used to measure temperature, that is, the degree of hotness or coldness 

 of a body. 



How expansion may indicate temperature. It has already been 

 learned that substances usually expand when heated and contract when 

 cooled. A flask filled with water, for instance, and having a stopper 

 through which a glass tube passes, can be used to show the expansion 

 produced by heat and the contraction by cold. But this flask and tube 

 make but a very rough temperature measurer. The water does not 

 get larger to the same amount for every equal addition of heat. Neither 

 is it very sensitive, that is to say, it does not show very small increases 

 in the degree of hotness or coldness, or, as the student must now learn 

 to say, it does not record very small differences of temperature, and for 

 a thermometer to be of value it must do this. Then, too, as every one 

 knows, if water is made very cold it becomes ice, which, being larger 

 than the water from which it is made, would crack the tube. For many 

 reasons, therefore, water is not a good thing to use in a thermometer. 



Choice of things to be used in a thermometer. 



1. The substance used should expand a great deal for a small increase 

 of temperature. 



Gases expand most, and solids least, for a given increase of tem- 

 perature. Liquids occupy a middle place. The most delicate ther- 

 mometers are therefore those where a gas, such as air, is the substance 

 which expands. But in common thermometers a liquid, either quick- 

 silver or spirits of wine, is used. Both these liquids expand a fair 

 amount for a given increase of temperature, and, to make this amount 

 of expansion as obvious as possible, they are used in fine threads by 

 making them expand into a tube with a fine very bore. 



2. If a liquid is used it should not change into a solid unless cooled very 

 much, nor into a gas unless heated very much. 



It is difficult to be sure of both these things in the same thermometer. 

 When a thermometer is required for measuring very low temperature 



