INTERNAL, OR MOLECULAR FORCES. 29 



wTiat is 56. Iron may be made to cohere to iron by 



■Welding? beating the metal to a high degree, and ham- 

 mering the two pieces together. The particles are thus 

 driven into such intimate contact, that they cohere and 

 form one uniform mass. This property is called Weld- 

 ing, and only belongs to two metals, iron and platinum. 



J-RACTICAL QUESTIONS ON THE INTERNAL, OR MOLECULAR 



FORCES. 



1. In ■what respect does a gas diffeb from a liquid ? 



A liquid, like ■water, milk, syrup, etc., can be made to flo^w regularly down 

 a slope., or an inclined plane, but a gas can not. 



2. Why is a bar of raox stronger than a bar of ivoo© of the same size ? 



Because the cohesion existing between the particles of iron ia greater than 

 that existing between the particles of A^-ood. 



3. Why are the particles of a liquid more easily separated than those of a eoLm? 

 Because the cohesive attraction which binds together the particles of a liquid 



ia much loss strong than that which binds together the particles of a solid. 



4. Why TvlU a small needle, carefully laid upon the surface of water, float ? 

 Because its ■v\^eight is not sufficient to overcome the cohesion of the particles 



of water constituting the surface ; consequently, it can not pass through them 

 and sink. 



5. If Tou drop watrr and laudanum from the same vessel, why will sixtt drops of the 

 waAdT fll the same measure asoxE iiundeed drops of laudanum? 



The cohesion between the particles of the two liquids is different, being 

 greatest in the water. Consequently, the number of particles which will ad- 

 here together to constitute a drop of water, is greater than in the drop of 

 laudanum. 



6. Why is the prescription of medicine by r>K0P8 an unsafe method ? 



Because, not only do drops of fluid from the same vessel, and often of tho 

 same fluid from different vessels, dilfer in size, but also drops of the same fluid, 

 to the extent of a third, from different parts of the hp of the same vessel 

 I 7. Why arc cements and mortars used to fasten bricks and stone together? 

 . Because the adhesive attraction between the particles of brick and stone 

 and the particles of mortar, is so strong, that they unite to form one soUd 

 mass. 



8. How may the efficacy of a locomotive engine be said to depend upon the force of 

 adhesion ? 



If there were no adhesion, or even insufficient adhesion, between the tire 

 of the driving-wheel of the locomotive, and the rails upon which it presses, 

 the wheel would turn without advancing. 



This actually happens whea the rails are greasy, or covered with frost and 



