The terms, variables, and associated units for the four principal measurable quantities of gases were presented and discussed in the previous section of this chapter. While studying each of these quantities independently, scientists discovered that that altering the experimental parameters for one of these variables impacted the values of the remaining measurements. Therefore, scientists designed and executed a series of additional experiments, in order to explore the qualitative and quantitative impacts of varying each of these measurable quantities.
The results of an experiment in which only two quantities are allowed to change can be qualitatively described using a proportion. Two variables are directly, or linearly, proportional if their corresponding values both increase or both decrease at the same relative rate. In other words, if the value of one quantity increases, the other must also increase. Alternatively, if one numerical quantity decreases, the other must also decrease. Mathematically, dividing two directly proportional quantities results in the derivation of a constant value. In contrast, if the values of two quantities change in opposite directions, those variables are indirectly, or inversely, proportional to one another. In this type of relationship, if one numerical value increases, the other must decrease at the same relative rate. Mathematically, a constant value results upon multiplying two quantities that are indirectly proportional.
In order to establish which type of proportion describes the relationship between each combination of gas-related variables, scientists limited their initial trials to the investigation of two of the four measurable quantities of gases. The measurements that were associated with two variables were held constant, so that any experimental change in the third quantity could only influence the value of the fourth. The equations that were derived upon the completion of these experimental trials, which are described in the following paragraphs, became collectively known as the Gas Laws.