When a body is heated about the room temperature and then left to itself, the body starts cooling. It is found that the rate of cooling is proportional to the difference of temperature between the hot body and its surrounding, provide a difference of temperature is small.
This law applies only if there is forced convection. This is newton's law of cooling. The rate of cooling also depends upon the area and nature of surface of hot vessel, but not on the nature of liquid inside. If water and another liquid are allowed to cool under similar conditions, their rate of cooling will be equal.
In this way we can find the specific heat of a liquid. For this, the apparatus consist of a test tube which contain water or liquid. It is suspended in a can which is surrounded by another can. This space between the can is filled with ice.
A cover is put on them with holes for thermometer. First, we put hot water of mass in test. The water is stairs and its temperature is observed after every minutes for 20 minutes. The temperature is plotted against time and we get the graph AB.
Then repeat the same process using another liquid of same mass and specific heat. Now we get the graph AC.the graph AB and AC are same.We find time interval for water and liquids to cool.