Final answer:
When testing Boyle's law using a manometer, temperature and the amount of gas must remain constant to accurately assess the inverse relationship between pressure and volume.
Step-by-step explanation:
The two variables that are held constant when testing Boyle's law in a manometer are temperature and the amount of gas (usually expressed in moles). To true investigate the relationship defined by Boyle's law, which describes the inverse relationship between pressure and volume, it is essential that the temperature remains the same throughout the experiments. Boyle's law is mathematically expressed as P₁V₁ = P₂V₂, meaning that the product of pressure and volume for a given sample of gas at constant temperature always equals the same value, denoting that these two properties are inversely proportional.