How was the absolute zero of temperature determined

Charles (1746-1823) was an aristocratic amateur scientist of the 18th century. He already knew that the volume V of a gas increased with increasing temperature T, and was determined to find a relationship between these variables. 

The law that now bears his name can be stated as, The ratio of volume and temperature for a fixed mass of gas remains constant , provided the external pressure is not altered. 

According to Charles s law, a linear relationship exists between V and T (at constant pressure p). 

This is one form of Charles s law . (Charles s law is also called Gay-Lussac s law .) Alternatively, we could have multiplied both sides of Equation (1.4) by T, and rewritten it as 

A straight line will always have an equation of the type y = mx + c, where m is the gradient and c is the intercept on the y-axis (i.e. when the value of x = 0). 

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