The discovery of inert gases gave a death blow to the Newlands Law.

Explanation:
The Newlands Law, also known as the Law of Octaves, was proposed by John Newlands in 1864. According to this law, when elements were arranged in order of their increasing atomic weights, every eighth element showed similar properties. This periodicity of properties was similar to the musical octaves, hence the name "Law of Octaves."

However, the discovery of inert gases posed a challenge to the Newlands Law and eventually led to its demise. Here's why:

1. Inert Gases:
- Inert gases, also known as noble gases, are a group of elements that include helium, neon, argon, krypton, xenon, and radon.
- These elements are characterized by their extremely low reactivity and stable electron configurations.
- Inert gases were discovered in the late 19th century by scientists such as Sir William Ramsay and Lord Rayleigh.

2. Breaking the Pattern:
- The discovery of inert gases broke the pattern of Newlands' Law.
- These gases did not fit into the existing arrangement of elements based on their atomic weights.
- The inert gases were placed at the end of the periodic table, separate from other elements, and did not exhibit a repeating pattern every eighth element.

3. Failure of the Law:
- The inclusion of inert gases disrupted the periodicity proposed by the Law of Octaves.
- It became evident that the pattern observed by Newlands was not consistent throughout the periodic table.
- The discovery of inert gases highlighted the need for a more comprehensive and accurate periodic table.

4. Mendeleev's Periodic Table:
- Dmitri Mendeleev, a Russian chemist, is credited with developing the modern periodic table in the 1860s.
- Mendeleev's table arranged elements based on their atomic weights but also took into account their chemical properties.
- The inert gases found their place in the noble gas group, which was added as a new group in Mendeleev's periodic table.

Conclusion:
The discovery of inert gases challenged the Newlands Law by breaking the pattern of periodicity. It highlighted the need for a more accurate and comprehensive periodic table, which was later developed by Mendeleev. The inclusion of the inert gases in the noble gas group further solidified the understanding of element arrangement based on their properties and atomic numbers.