CHAPTER XVII. 



33. OBSERVATION OF THE PROPERTIES OF 

 SUBSTANCES. 



BEFORE the changes which a body undergoes when acted on by heat, 

 or by other substances are noted, it is essential to examine it carefully 

 and to make an accurate record of the original properties of the body. 



i. Describe the properties of any three very familiar substances, say : 

 glass, salt and mustard. 



Description of substances. Comparing the description of any 

 substance given by different members of a class, it is noticed that one 

 pupil leaves out properties which another gives, and vice versa. If 

 all the properties mentioned are written down it is noticed that they 

 can be placed under definite headings, and in order to make certain 

 of an accurate description, in every case the properties decided on 

 must not only be recorded, but recorded in a definite order. 



Colour. One of the first things to notice about any body is its 

 colour. Where possible the exact shade should be given. Some 

 substances such as glass and water can be seen through and they are 

 said to be transparent. (Note that although solid glass is colourless, 

 when it is ground into a powder it appears to be white.) Other 

 substances allow light to pass through them although they cannot 

 be seen through and such substances are said to be translucent. 

 Things which do not allow light to pass through them are said to be 

 opaque. 



Taste Taste is a most important property and many substances 

 can be recognised by it alone. Il should be distinctly understood that 

 in no circumstances should a substance be tasted without the permission 

 of the teacher, as it may be a deadly poison. Many poisons resemble 

 quite common substances in appearance. In any case only a very 

 small quantity of a substance should be taken into the mouth. Sub- 

 stances without taste are said to be tasteless ; other taste sensations 

 are sourness or acidity, sweetness, bitterness, burning, etc. 



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